Fix sound when build against FFmpeg 1.1

commits taken from xbmc :
dcd820ffed - [ffmpeg] Convert from av_audio_convert API to the swresample one.
c8f8fb7913 - [rbp] fixed omx audio codec compile after dcd820f
7a9b6f5934 - [rbp/omxplayer] fixed only reinitalize convert context when sample format changed