RFS: mgba/0.8.4+dfsg-1

I received a bug report that mgba FTBFS (#973069). I identified the root 
cause as a change in libzip (#973280), but the libzip maintainers have 
not answered yet, so I added a workaround in mgba for now, which I hope 
will be temporary.

I also took the opportunity to update to the latest upstream bugfix 
release (0.8.4).

The build passes in salsa-ci and I have tested the package myself under 
unstable.

Would you upload mgba to unstable, please?

thank you,
Ryan