avformat: adapt size type change

When compiling with FFmpeg 5:

    ../../modules/demux/avformat/demux.c: In function ‘Demux’:
    ../../modules/demux/avformat/demux.c:817:84: warning: passing argument 3 of ‘av_packet_get_side_data’ from incompatible pointer type [-Wincompatible-pointer-types]
      817 |     uint8_t *side_data = av_packet_get_side_data( &pkt, AV_PKT_DATA_NEW_EXTRADATA, &side_data_size );
          |                                                                                    ^~~~~~~~~~~~~~~
          |                                                                                    |
          |                                                                                    int *

Refs ffmpeg/14040a1d913794d9a3fd6406a6d8c2f0e37e0062 (announcement)
Refs ffmpeg/ef6a9e5e311f09fa8032974fa4d0c1e166a959bb (change)

modules/demux/avformat/demux.c

@@ -820,7 +820,11 @@ static int Demux( demux_t *p_demux )
     }
 
     // handle extra data change, this can happen for FLV
+#if LIBAVUTIL_VERSION_MAJOR < 57
     int side_data_size;
+#else
+    size_t side_data_size;
+#endif
     uint8_t *side_data = av_packet_get_side_data( &pkt, AV_PKT_DATA_NEW_EXTRADATA, &side_data_size );
     if( side_data_size > 0 ) {
         // ignore new extradata which is the same as previous version