Add human-readable names for signal_ns

Change-Id: Id664355eb8305fb287e4dae0800fb20dc2f9b8cd
Related: SYS#3610
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index 130d8c0..9cebbd6 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -203,6 +203,8 @@
 	S_NS_MISMATCH, /* got an unexpected IE (sets msg, pdu_type, ie_type) */
 };
 
+extern const struct value_string gprs_ns_signal_ns_names[];
+
 struct ns_signal_data {
 	struct gprs_nsvc *nsvc;
 	struct gprs_nsvc *old_nsvc;