Add missing _CFLAGS and _LIBS

These missing pieces go unnoticed if dependencies are not installed in
distinct directories.

Change-Id: If8d57b72f63d79cc0d8efba7466c6ec177207cbb
diff --git a/openbsc/tests/sndcp_xid/Makefile.am b/openbsc/tests/sndcp_xid/Makefile.am
index 99b9d1a..d09c41b 100644
--- a/openbsc/tests/sndcp_xid/Makefile.am
+++ b/openbsc/tests/sndcp_xid/Makefile.am
@@ -15,6 +15,7 @@
 	$(LIBOSMOGB_LIBS) \
 	$(LIBCARES_LIBS) \
 	$(LIBCRYPTO_LIBS) \
-	-lgtp -lrt -lm
+	$(LIBGTP_LIBS) \
+	-lrt -lm