Please upgrade libsdl-mixer-2 to get MP3 support fixed

[Tobias Leich]

Hi, in libsdl2-mixer-2.0-0 exists a bug that disallows mp3 support when
libmad is used as the mp3 backend.
Since that is the default on debian, it affects all debian-esque systems.

This is the fix: https://hg.libsdl.org/SDL_mixer/rev/1aca2b6d570f

Sorry that I report this on this way. I have problems doing it the
'right' way since I run ubuntu...

Cheers, Tobias