[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
```
Any idea why it fails:
Run till exit from #0  avformat_open_input (ps=0x7fffffffd440, filename=0x1bfdb18 "/dev/shm/osc/home/abuild/rpmbuild/BUILD/mixxx/src/test/id3-test-data/cover-test.mp3", fmt=0x0, options=0x0)
    at libavformat/utils.c:543
No URL Protocols are registered. Missing call to av_register_all()?
0x00000000009d6fba in SoundSourceFFmpeg::parseCoverArt (this=0x1b22a20) at src/soundsourceffmpeg.cpp:645
645         if (avformat_open_input(&FmtCtx, m_qFilename.toLocal8Bit().constData(), NULL, NULL) != 0) {
Value returned is $1 = -1330794744