Fix regression in tr_GSUP_PROC_SS_ERR during inter-MSC-ho

In Change-Id I7d76c982ad4e198534fa488609c41e8892b268ab we merged
various changes to GSUP_Templates.ttcn, mostly related to adding
the tr_GSUP_IE_Message_Class IE.

In most caes, the specified MessageClass is correct.  However in
tr_GSUP_PROC_SS_ERR we accidentially used OSMO_GSUP_MESSAGE_CLASS_SMS
instead of OSMO_GSUP_MESSAGE_CLASS_USSD.

This makes TC_lu_and_ss_session_timeout pass again.

Change-Id: I04fa1be24ec63ed1eb767a33de0297722b4366f1
diff --git a/library/GSUP_Types.ttcn b/library/GSUP_Types.ttcn
index 44b9f26..df1d7a9 100644
--- a/library/GSUP_Types.ttcn
+++ b/library/GSUP_Types.ttcn
@@ -1174,7 +1174,7 @@
 		tr_GSUP_IE_Cause(cause),
 		tr_GSUP_IE_SessionId(sid),
 		tr_GSUP_IE_SessionState(state),
-		tr_GSUP_IE_Message_Class(OSMO_GSUP_MESSAGE_CLASS_SMS)
+		tr_GSUP_IE_Message_Class(OSMO_GSUP_MESSAGE_CLASS_USSD)
 	}
 );