Add libFLAC.so.8 to the flatpak environment

I've found multiple Linux games on GOG.com that won't start because `libFLAC.so.8` is missing inside the Minigalaxy flatpak, e.g. "Ion Fury" and "The Original Strife: Veteran Edition". This library is present on all Linux machines I own, so it seems to be commonly installed, but inside the flatpak environment it's apparently unavailable. I suggest it should be added to the flatpak environment. I don't know how exactly this is best done since apt seems to be strangely missing inside the flatpak sandbox, but I assume `libFLAC.so.8` doesn't have many dependencies so if all else fails you could just most likely just copy the library file from any regular e.g. Ubuntu system and copy it into the flatpak image that way.

**Affected flatpak version:** 1.3.1 minigalaxy flatpak