Add control interface

Only generation of TRAP messages over Control Interface is supported so
far.

Note: requires corresponding version of libosmoctrl.

Change-Id: Ia76f841d2c9cd14394e9316fcd39f4060e23c898
Related: OS#1646
diff --git a/gtp/gtp.h b/gtp/gtp.h
index 539e255..fd138cc 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -12,6 +12,8 @@
 #ifndef _GTP_H
 #define _GTP_H
 
+#include <osmocom/ctrl/control_if.h>
+
 #define GTP_MODE_GGSN 1
 #define GTP_MODE_SGSN 2
 
@@ -245,7 +247,7 @@
 
 	unsigned char restart_counter;	/* Increment on restart. Stored on disk */
 	char *statedir;		/* Disk location for permanent storage */
-
+	struct ctrl_handle *ctrl;	/* Control Interface */
 	struct queue_t *queue_req;	/* Request queue */
 	struct queue_t *queue_resp;	/* Response queue */