Fix building virnetserverclientmock with MinGW

Signed-off-by: NPavel Fedin <p.fedin@samsung.com>
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

tests/Makefile.am

@@ -931,6 +931,7 @@ virnetserverclientmock_la_SOURCES = \
 	virnetserverclientmock.c
 virnetserverclientmock_la_CFLAGS = $(AM_CFLAGS)
 virnetserverclientmock_la_LDFLAGS = $(MOCKLIBS_LDFLAGS)
+virnetserverclientmock_la_LIBADD = $(GNULIB_LIBS)
 
 if WITH_GNUTLS
 virnettlscontexttest_SOURCES = \