hls: Call avformat_find_stream_info() on the chained demuxers

This allows the chained demuxer (or more precisely, the lavf
utility code) to better fill in timestamps on packets from
these, especially for cases where one stream is a raw ADTS
stream.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  hls: Call avformat_find_stream_info() on the chained demuxers

Conflicts:
	libavformat/hls.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>