lavf/oggparseopus: Setting seek_preroll in AVCodecContext

Setting seek_preroll value in AVCodecContext for Opus streams
embedded in ogg container.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>