test-nss-hosts: do not call seccomp functions if HAVE_SECCOMP is not set

test-nss-hosts: do not call seccomp functions if HAVE_SECCOMP is not set

Otherwise, the build fails:

```
  Features
...
    disabled                                 : ACL, SECCOMP, ...
...
[1592/2115] Compiling C object test-nss-hosts.p/src_test_test-nss-hosts.c.o
FAILED: test-nss-hosts.p/src_test_test-nss-hosts.c.o
...
In file included from ../src/test/test-nss-hosts.c:27:
../src/test/test-nss-hosts.c: In function ‘run’:
../src/test/test-nss-hosts.c:497:43: error: implicit declaration of function ‘seccomp_filter_set_add_by_name’ [-Werror=implicit-function-declaration]
  497 |                                 ASSERT_OK(seccomp_filter_set_add_by_name(filter, /* add = */ true, s));
      |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:497:43: warning: nested extern declaration of ‘seccomp_filter_set_add_by_name’ [-Wnested-externs]
  497 |                                 ASSERT_OK(seccomp_filter_set_add_by_name(filter, /* add = */ true, s));
      |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:35: error: implicit declaration of function ‘seccomp_load_syscall_filter_set_raw’ [-Werror=implicit-function-declaration]
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:35: warning: nested extern declaration of ‘seccomp_load_syscall_filter_set_raw’ [-Wnested-externs]
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:71: error: ‘SCMP_ACT_ALLOW’ undeclared (first use in this function)
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                                                       ^~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:71: note: each undeclared identifier is reported only once for each function it appears in
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                                                       ^~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:95: error: implicit declaration of function ‘SCMP_ACT_ERRNO’ [-Werror=implicit-function-declaration]
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                                                                               ^~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-nss-hosts.c:498:95: warning: nested extern declaration of ‘SCMP_ACT_ERRNO’ [-Wnested-externs]
  498 |                         ASSERT_OK(seccomp_load_syscall_filter_set_raw(SCMP_ACT_ALLOW, filter, SCMP_ACT_ERRNO(ENOSYS), /* log_missing = */ true));
      |                                                                                               ^~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
cc1: some warnings being treated as errors
```

Fixes 788b3e030e717404cbb65b4261bff2ef7ddb1b90