[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 build with ```-Wall``` and according to the GCC manual, that implies ```-Wreturn-type```.