gsup: Add osmo_gsup_get_err_msg_type() function

This function can be used to resolve the error message type for
a given message type.  Can be used by generic error handlers that
work for any incoming message type.

Change-Id: Ic637bec53dd7fe3ec83da99b49b4eae34d5602b2
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 0a6742c..f98f7c9 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -469,6 +469,7 @@
 osmo_gsup_encode;
 osmo_gsup_decode;
 osmo_gsup_message_type_names;
+osmo_gsup_get_err_msg_type;
 
 osmo_oap_encode;
 osmo_oap_decode;