Re-add the msc 0->mgw config line since the "latest" version of osmo-bsc still uses it. This was probably caused by fixing fallout from osmo-bsc.git commit da4af65a51ee8b8d7b380330c0301de293347563 in master.
The config line is ignored in master so should be safe to keep until a new osmo-bsc version is released.
