[Build Fix] Implement SoundSourceFFmpeg::parseCoverArt().

Otherwise build fails with:
```
lin64_build/soundsourceffmpeg.o:(.rodata._ZTV17SoundSourceFFmpeg[vtable for SoundSourceFFmpeg]+0x48): undefined reference to `SoundSourceFFmpeg::parseCoverArt()'
collect2: ld returned 1 exit status
scons: *** [lin64_build/mixxx] Error 1
```
We have an older codebase where some files do not correspond to our coding style. In general we try to clean up files as we touch them.

You can check the coding guidelines here
http://www.mixxx.org/wiki/doku.php/coding_guidelines