[SysApps] Expose ffmpeg function signatures needed by Device Capabilities API

We need these functions in to get the list of Codecs available in the
system for getAVCodecs(). If we ever fork ffmpeg, we can just remove
this patch and add the signatures directly at the ffmpeg's signature
list.
Merge pull request #90 from tmpsantos/ffmpeg_sigs

[SysApps] Expose ffmpeg function signatures needed by Device Capabilities API