ext/ffmpeg/: Add a stream-header flag to set stream-header caps on some streams, such as (in this case) flv (fixes #3...

Original commit message from CVS:
Reviewed by:  Ronald S. Bultje  <rbultje@ronald.bitfreak.net>
* ext/ffmpeg/gstffmpeg.h:
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_loop):
* ext/ffmpeg/gstffmpegprotocol.c: (gst_ffmpegdata_open),
(gst_ffmpegdata_write):
Add a stream-header flag to set stream-header caps on some streams,
such as (in this case) flv (fixes #309051).