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 https://gitlab.freedesktop.org/gstreamer/gst-libav/-/issues/85 Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-libav/-/merge_requests/127>