ext/ffmpeg/gstffmpegdec.c: Include optional parser if supported (currently only MPEG-1/-2 video). This fixes playback...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_init),
(gst_ffmpegdec_close), (gst_ffmpegdec_open), (gst_ffmpegdec_chain):
Include optional parser if supported (currently only MPEG-1/-2
video). This fixes playback of MPEG-1/-2 videos using
ffdec_mpeg2video.