ttcn3-bsc: Enable OSMUX feature in osmo-bts-omldummy
This feature is used to indicate to the BSC that the BTS supports Osmux.
Requires: libosmocore 18c6a8183f92915e77368ecffb1cbf7f555453a3
Related: SYS#5987
Change-Id: Ia402b7514b636750442d0859d5ebc3fcad67dd9f
