tests: avoid dlsym mocking on mingw

tests: avoid dlsym mocking on mingw

I got a build failure when cross-compiling to mingw with the
mingw64-dbus package installed:

  CC       virmockdbus_la-virmockdbus.lo
../../tests/virmockdbus.c:29:6: error: 'dbus_connection_set_change_sigpipe' redeclared without dllimport attribute: previous dllimport ignored [-Werror=attributes]
 VIR_MOCK_STUB_VOID_ARGS(dbus_connection_set_change_sigpipe,
      ^
../../tests/virmockdbus.c:33:18: error: 'dbus_bus_get' redeclared without dllimport attribute: previous dllimport ignored [-Werror=attributes]
 VIR_MOCK_STUB_RET_ARGS(dbus_bus_get,
...

Well duh - mingw lacks dlopen and friends, even if it can support
dbus.  A similar failure occured in virsystemdtest.c; but in that
file, we know that systemd is a Linux-only concept.

* tests/virmockdbus.c: Cripple on mingw.
* tests/virsystemdtest.c: Cripple on non-Linux.

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