Fix system ffmpeg integration for Linux OSes.

[DistantThunder]

Issue: #7955

It finally occured to me that it the "old_pix_fmts.h" header file didn't exist in the upstream FFMPEG source and that it was there that the old variable names that ppsspp use were defined.

This pull allows for proper compilation when using the local system ffmpeg libraries. It would fail otherwise, throwing something allong the lines of:

ppsspp/Core/HLE/sceMpeg.cpp:776:21: error: ‘PIX_FMT_RGBA’ was not declared in this scope
  pmp_want_pix_fmt = PIX_FMT_RGBA;
                     ^
ppsspp/Core/HLE/sceMpeg.cpp:779:45: error: ‘CODEC_ID_H264’ was not declared in this scope
  AVCodec * pmp_Codec = avcodec_find_decoder(CODEC_ID_H264);

[hrydgard]

Trying a simpler alternative: 33c3963

Does that solve the problem?