Fix ByteIOContext init on recent libavformat

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/avformat/demux.c

@@ -223,6 +223,9 @@ int OpenDemux( vlc_object_t *p_this )
        */
        p_sys->url.is_streamed = 1;
        p_sys->io.is_streamed = 1;
+#if defined(AVIO_SEEKABLE_NORMAL)
+       p_sys->io.seekable = 0;
+#endif
     }