Revert broken part of [6f653c5a2ba30008d62ab589ff5686df8a29ef72].

This was obviously untested as it breaks external ffmpeg build and only changes this code path.
xbmc/cores/dvdplayer/DVDDemuxers/DVDDemuxers.a(DVDDemuxFFmpeg.o): In function `DllAvFormat::av_read_frame_flush(AVFormatContext*):
/home/alexis/xbmc/lib/DllAvFormat.h:121: undefined reference to
`ff_read_frame_flush'

Even if it did not break, the change is wrong anyway.
Rename our local copy of av_read_frame_flush to xbmc_read_frame_flush as suggested by elupus on PR2633.

That way it makes it clear the function is from xmbc and not ffmpeg.
Merge pull request #2633 from aballier/mess

Revert broken part of [6f653c5a2ba30008d62ab589ff5686df8a29ef72].