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/libosmogsm.map b/src/gsm/libosmogsm.map
index 8b7a164..cdcbcc9 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -555,6 +555,7 @@
 sgsap_ie_tlvdef;
 
 osmo_rat_type_names;
+osmo_lu_type_names;
 
 local: *;
 };