libgstcheck: export gst_consistency_checker_add_pad()

Fix build of the adder unit test in -base again.
diff --git a/libs/gst/check/Makefile.am b/libs/gst/check/Makefile.am
index 8dcf66d..6115d99 100644
--- a/libs/gst/check/Makefile.am
+++ b/libs/gst/check/Makefile.am
@@ -83,6 +83,7 @@
 	gst_check_teardown_pad_by_name \
 	gst_check_teardown_sink_pad \
 	gst_check_teardown_src_pad \
+	gst_consistency_checker_add_pad \
 	gst_consistency_checker_new \
 	gst_consistency_checker_reset \
 	gst_consistency_checker_free