DVDCodecs.h: Do not include avcodec.h directly.

If at all it should include lib/DllAvCodec.h but this does not seem to be needed here.
DVDAudio.h: Include DllAvCodec.h which does the right thing to get the definitions instead of manually including avcodec.h.
DVDStreamInfo.h: Include DllAvCodec.h which does the right thing to get the definitions instead of manually including avcodec.h.
DVDDemux.h: Include DllAvCodec.h which does the right thing to get the definitions instead of manually including avcodec.h.
DVDDemuxPVRClient.h: No need to manually include avformat.h since we already include DllAvFormat.h
DVDDemuxUtils.cpp: Include DllAvCodec.h which does the right thing to get the definitions instead of manually including avcodec.h.
DVDDemuxFFmpeg.cpp: Do not exclude OTF fonts in FFmpeg attachements with external ffmpeg. This is supported in ffmpeg-1.0 which happens to be the minimum required version for building xbmc.
Merge pull request #3007 from aballier/ffmpeg_includes

Kill USE_EXTERNAL_FFMPEG usage in xbmc/* code