Make f_gen_tr_ss_ies() work for both 'latest' and 'master'

This fixes the partial revert of c43c8cd275b4f71b0bc6a50fc81564c812d5b406
to work for both situtions:  Messages that have the OSMO_GSUP_MESSAGE_CLASS_USSD
and messages that don't.

The particular implementation is rather ugly, but we're waiting for
a response to https://www.eclipse.org/forums/index.php/t/1098847/
on how to solve this kind of problem in a more elegant way.  Meanwile,
we make it work first.

Change-Id: Ibf137de6a41aaa43894cc0b6da8341ceb88b0756
1 file changed