add osmo_lu_type_names[], osmo_lu_type_name()

Move lupd_names[] from osmo-msc to libosmo-gsm.

Change-Id: Ica25919758ef6cba8348da199b0ae7e0ba628798
diff --git a/src/gsm/gsm48.c b/src/gsm/gsm48.c
index 853ea51..099b85d 100644
--- a/src/gsm/gsm48.c
+++ b/src/gsm/gsm48.c
@@ -1137,4 +1137,12 @@
 	return gh;
 }
 
+const struct value_string osmo_lu_type_names[] = {
+	{ GSM48_LUPD_NORMAL, "NORMAL" },
+	{ GSM48_LUPD_PERIODIC, "PERIODIC" },
+	{ GSM48_LUPD_IMSI_ATT, "IMSI-ATTACH" },
+	{ GSM48_LUPD_RESERVED, "RESERVED" },
+	{}
+};
+
 /*! @} */