vty: Print MM state and RAN type in show mm-context

Value presviosuly printed as MM state is actually usually refereed as
GMM state, so rename it.

Change-Id: Ia06c53f0cd2a6348237ed3abeb9367d965745aba
diff --git a/include/osmocom/sgsn/gprs_sgsn.h b/include/osmocom/sgsn/gprs_sgsn.h
index 8f16c5b..8e8b762 100644
--- a/include/osmocom/sgsn/gprs_sgsn.h
+++ b/include/osmocom/sgsn/gprs_sgsn.h
@@ -104,6 +104,7 @@
 	MM_CTX_T_GERAN_Iu,
 #endif
 };
+extern const struct value_string sgsn_ran_type_names[];
 
 struct service_info {
 	uint8_t type;