avformat/HLS: modified EXT-X-MEDIA-SEQUENCE header in order to follow the HLS standard

When using the wrap option the EXT-X-MEDIA-SEQUENCE header of the playlist file was reset instead of being incremented.
It is now incremented by one for every media URI removed from the playlist file as the standard states.

Signed-off-by: Willy Aubry <waubry@viotech.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>