gb: Import mnl.h iif --enable-libmnl

Change-Id: Ic0100493a9256e9c30a4bbb92be404270a8b9393
diff --git a/src/gb/gprs_ns2_fr.c b/src/gb/gprs_ns2_fr.c
index a651e1d..8befe1c 100644
--- a/src/gb/gprs_ns2_fr.c
+++ b/src/gb/gprs_ns2_fr.c
@@ -50,9 +50,12 @@
 #include <osmocom/core/select.h>
 #include <osmocom/core/socket.h>
 #include <osmocom/core/talloc.h>
-#include <osmocom/core/mnl.h>
 #include <osmocom/gprs/gprs_ns2.h>
 
+#ifdef ENABLE_LIBMNL
+#include <osmocom/core/mnl.h>
+#endif
+
 #include "config.h"
 #include "common_vty.h"
 #include "gprs_ns2_internal.h"