Fix stow-enabled jenkins build failure

The error is:
CC       gtp-kernel.o
gtp-kernel.c:19:26: fatal error: libgtpnl/gtp.h: No such file or directory
 #include <libgtpnl/gtp.h>
                          ^
compilation terminated.

Fix it by using proper CFLAGS/LIBS for libgtpnl.

Change-Id: I5a24076778ea3ce263ac27211a6f45f935155b33
diff --git a/ggsn/Makefile.am b/ggsn/Makefile.am
index 8a468a9..6b2232f 100644
--- a/ggsn/Makefile.am
+++ b/ggsn/Makefile.am
@@ -7,8 +7,8 @@
 osmo_ggsn_LDADD = @EXEC_LDADD@ -lgtp -L../gtp ../lib/libmisc.a $(LIBOSMOCORE_LIBS) $(LIBOSMOCTRL_LIBS) $(LIBOSMOVTY_LIBS)
 
 if ENABLE_GTP_KERNEL
-AM_CFLAGS += -DGTP_KERNEL
-osmo_ggsn_LDADD += -lgtpnl
+AM_CFLAGS += -DGTP_KERNEL $(LIBGTPNL_CFLAGS)
+osmo_ggsn_LDADD += $(LIBGTPNL_LIBS)
 endif
 
 osmo_ggsn_DEPENDENCIES = ../gtp/libgtp.la ../lib/libmisc.a