hlssink* elements could be finalized without EOS event, and in that case
the final playlist might not include the EXT-X-ENDLIST tag.
Since missing ENDLIST tag means it's live stream, but we did't intend it,
hlssink* elements should put the tag at the end.
Remove playlist_str GString variable from GstM3U8Playlist struct,
since it's only used temporarily in playlist_render(). Might just
as well keep it local then.
