harness: make sure g_assert() statements are always active

We have code with side effects inside g_assert()s, so make
sure those are always enabled here (they might otherwise
get disabled for release builds).
diff --git a/libs/gst/check/Makefile.am b/libs/gst/check/Makefile.am
index a08c459..84a1c5a 100644
--- a/libs/gst/check/Makefile.am
+++ b/libs/gst/check/Makefile.am
@@ -13,6 +13,7 @@
 	gsttestclock.c
 
 libgstcheck_@GST_API_VERSION@_la_CFLAGS = $(GST_OBJ_CFLAGS) \
+	-UG_DISABLE_ASSERT \
 	-I$(top_builddir)/libs \
 	-I$(top_builddir)/libs/gst/check \
 	-I$(top_builddir)/libs/gst/check/libcheck