tests: Link virtestmock with probes

I've encountered this error while trying out this feature on some
systems:

  $ VIR_TEST_FILE_ACCESS=1 ./virhashtest \
 libvirt.git/tests/.libs/lt-virhashtest: \
symbol lookup error: libvirt.git/tests/.libs/virtestmock.so: \
undefined symbol: libvirt_event_poll_purge_timeout_semaphore

Problem is, linking just libvirt_utils to virmock.la is not
enough. We might need to link libvirt_probes.lo too.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -1178,6 +1178,7 @@ virtestmock_la_CFLAGS = $(AM_CFLAGS)
 virtestmock_la_LDFLAGS = $(MOCKLIBS_LDFLAGS)
 virtestmock_la_LIBADD = \
 	$(MOCKLIBS_LIBS) \
+	$(PROBES_O) \
 	../src/libvirt_util.la
 else ! WITH_LINUX
 	EXTRA_DIST += virusbtest.c virusbmock.c \