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/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index cff17dd..cb09979 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -99,8 +99,8 @@
 	$(LIBOSMOABIS_LIBS) \
 	$(LIBCARES_LIBS) \
 	$(LIBCRYPTO_LIBS) \
+	$(LIBGTP_LIBS) \
 	-lrt \
-	-lgtp \
 	-lm \
 	$(NULL)
 if BUILD_IU