omxvideodec: allow to start decoder on HEADER buffer
omxvideodec: allow to start decoder on HEADER buffer

If the headers are sent in their own buffer
it won't have the SYNC_FRAME flag but we still
do want to start decoding rather than dropping it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-omx/-/merge_requests/49>

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx