avcodec/videotoolboxenc: Fixes non-B-Frame encoding

Sets vtctx->has_b_frames to 0 if the VideoToolbox compression
session will not emit B-frames (and, in consequence, no valid
DTSs). Required for the handling of invalid DTSs in
'vtenc_cm_to_avpacket' (line 2018ff) to work correctly and not
abort encoding with "DTS is invalid." when no B-frames are
generated.

Signed-off-by: NoHalfBits <ffmpeg-devel@fluthaus.com>
Signed-off-by: Rick Kern <kernrj@gmail.com>
