avformat/hlsenc: Respect `omit_endlist` flag in subtitle playlists

Ensure that when the `-hls_flags omit_endlist` option is set,
the `#EXT-X-ENDLIST` tag is also omitted from the `stream_vtt.m3u8`
subtitle playlist. This maintains consistency with the behavior
in other playlists when `omit_endlist` is specified.

Reviewed-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/hlsenc.c

@@ -1706,7 +1706,7 @@ static int hls_window(AVFormatContext *s, int last, VariantStream *vs)
             }
         }
 
-        if (last)
+        if (last && !(hls->flags & HLS_OMIT_ENDLIST))
             ff_hls_write_end_list(hls->sub_m3u8_out);
 
     }