build: skip systemd mock on non-Linux

build: skip systemd mock on non-Linux

A cross-compile to mingw failed:

  CC       virsystemdmock_la-virsystemdmock.lo
../../tests/virsystemdmock.c:29:6: error: 'dbus_connection_set_change_sigpipe' redeclared without dllimport attribute: previous dllimport ignored [-Werror=attributes]
 void dbus_connection_set_change_sigpipe(dbus_bool_t will_modify_sigpipe ATTRIBUTE_UNUSED)
      ^

But when you think about it, systemd is Linux-only, and even our
use of LD_PRELOAD to provide mock syscalls is Linux-only.

* tests/virsystemdmock.c: Avoid compilation outside Linux.

Signed-off-by: Eric Blake <eblake@redhat.com>