Add GTP message names

Change-Id: I65eb80db4bcdc6da4d267bef3b907d3f98942a2e
diff --git a/gtp/gtp.h b/gtp/gtp.h
index d189ded..8f8e293 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -12,6 +12,8 @@
 #ifndef _GTP_H
 #define _GTP_H
 
+#include <osmocom/core/utils.h>
+
 #define GTP_MODE_GGSN 1
 #define GTP_MODE_SGSN 2
 
@@ -85,6 +87,10 @@
 /* 242-254 For future use. */
 #define GTP_GPDU            255	/* G-PDU */
 
+extern const struct value_string gtp_type_names[];
+static inline const char *gtp_type_name(uint8_t val)
+{ return get_value_string(gtp_type_names, val); }
+
 /* GTP information element cause codes from 29.060 v3.9.0 7.7 */
 /*                                                            */
 #define GTPCAUSE_REQ_IMSI                   0	/* Request IMSI */