deps/BSSGP: Use specific commit hash

Using "master" doesn't really work as the local master branch already
exists even if we change the 'remote' to a different URL, see
https://gerrit.osmocom.org/c/osmo-ttcn3-hacks/+/21587

Change-Id: Id5f5ba52dbc9cd51dda2e91391c901b56756e2db
1 file changed