hlsdemux2: Ensure a discont will be set when resetting for lost sync

This is to ensures we inform the demuxer/parsers that what follows is not contiguous

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6610>

subprojects/gst-plugins-good/ext/adaptivedemux2/hls/gsthlsdemux-stream.c

@@ -1448,6 +1448,7 @@ lost_sync:
     stream->playlist = new_playlist;
     stream->playlist = gst_hls_media_playlist_ref (new_playlist);
     stream->playlist_fetched = TRUE;
+    stream->pending_discont = TRUE;
 
     gst_hls_demux_reset_for_lost_sync (demux);
   }