FFMpeg core failing to launch, missing libavcodec.so.56

FFMpeg core is failing to load on libavcodec.so.56. I am not sure if this is a bug in flathub packaging or if the core itself should provide the library. In any case, we can probably add it via a dependency while/if ffmpeg core is missing it.

> [ERROR] Failed to open libretro core: "/home/foo/.var/app/org.libretro.RetroArch/config/retroarch/cores/ffmpeg_libretro.so"
> [ERROR] Error(s): libavcodec-ffmpeg.so.56: cannot open shared object file: No such file or directory