avdemux: fix build with FFmpeg 4.4

Direct access to avstream->index_entries was removed
in favour of the newly added avformat_index_get_entry()
and friends.

Fixes gstreamer/gst-libav#85

Part-of: <gstreamer/gst-libav!129>
28 jobs for !129 with backport-mr-127-into-1.18-avdemux-fix-build-with-ffmpeg-4.4 in 67 minutes and 20 seconds (queued for 2 seconds)
merge request