libgtp: implement gtp_clear_queues to clear req/resp queue

Clearing the request and response queue is useful for debugging
to reset "some" state. Otherwise some tests will get un-expected
packets.

Change-Id: I279d1d7cbf5d37dd5609c2b968f317fe9a0e348d
diff --git a/gtp/gtp.h b/gtp/gtp.h
index ed594f1..5f35ab5 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -369,6 +369,8 @@
 				   struct pdp_t * pdp,
 				   uint8_t recovery));
 
+void gtp_clear_queues(struct gsn_t *gsn);
+
 /* Internal functions (not part of the API */
 
 extern int gtp_echo_req(struct gsn_t *gsn, int version, void *cbp,