ext/ffmpeg/gstffmpegdec.c: Update timestamp for every new buffer that we output. Improves audio playback for several ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_chain):
Update timestamp for every new buffer that we output. Improves
audio playback for several files that have multiple audio chunks
per buffer.