Stream timebase should be set using avpriv_set_pts_info, otherwise
avctx->pkt_timebase is not correct, leading to A/V desync.

Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Stephen Hutchinson <qyot27@gmail.com>
Signed-off-by: Diego Biurrun <diego@biurrun.de>