libaudit-util: fix build with audit disabled

libaudit-util: fix build with audit disabled

```
In file included from ../src/test/test-dlopen-so.c:21:
../src/test/test-dlopen-so.c: In function ‘run’:
../src/test/test-dlopen-so.c:53:23: error: implicit declaration of function ‘dlopen_libaudit’; did you mean ‘dlopen_libfido2’? [-Werror=implicit-function-declaration]
   53 |         ASSERT_DLOPEN(dlopen_libaudit, HAVE_AUDIT);
      |                       ^~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-dlopen-so.c:53:9: note: in expansion of macro ‘ASSERT_DLOPEN’
   53 |         ASSERT_DLOPEN(dlopen_libaudit, HAVE_AUDIT);
      |         ^~~~~~~~~~~~~
../src/test/test-dlopen-so.c:53:23: warning: nested extern declaration of ‘dlopen_libaudit’ [-Wnested-externs]
   53 |         ASSERT_DLOPEN(dlopen_libaudit, HAVE_AUDIT);
      |                       ^~~~~~~~~~~~~~~
../src/shared/tests.h:181:24: note: in definition of macro ‘ASSERT_OK’
  181 |                 typeof(expr) _result = (expr);                                                                  \
      |                        ^~~~
../src/test/test-dlopen-so.c:53:9: note: in expansion of macro ‘ASSERT_DLOPEN’
   53 |         ASSERT_DLOPEN(dlopen_libaudit, HAVE_AUDIT);
      |         ^~~~~~~~~~~~~

```

Follow-up for 4d8c5c657ae0829f93944a00302e7ce700913e54