Update ffmpeg revision to 19580 of the 0.5 branch

This cleans up licensing and makes it possible to build with LGPL code
only again.

Apart from that an opencore-amr encoder/decoder exists now but this
is not enabled as we don't want to use external libraries through ffmpeg
and have our own plugins for this already.

ffmpegrev

-FFMPEG_REVISION=18939
+FFMPEG_REVISION=19580
 FFMPEG_CO_DIR=gst-libs/ext/ffmpeg
 FFMPEG_SVN=svn://svn.ffmpeg.org/ffmpeg/branches/0.5
 # Because ffmpeg trunk checks out libswscale via an svn:externals, checking