gtp: Add missing headers

Those headers are using types defined in other places (like sockaddr_in)
and don't explicitly include them, which makes future queue_test fail.

Change-Id: I65e12a067d89ef71be3719636b64f4d93ea73cc4

gtp/pdp.h

@@ -14,6 +14,7 @@
 #define _PDP_H
 
 #include <stdbool.h>
+#include <netinet/in.h>
 
 #include <osmocom/core/defs.h>
 

gtp/queue.h

@@ -17,6 +17,8 @@
 #ifndef _QUEUE_H
 #define _QUEUE_H
 
+#include "gtp.h"
+
 #define QUEUE_DEBUG 0		/* Print debug information */
 
 #define QUEUE_SIZE 1024		/* Size of retransmission queue */