[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
```
https://www.ffmpeg.org/doxygen/2.3/group__lavf__decoding.html#gae804b99aec044690162b8b9b110236a4

You should close the files after you have parsed them.