avformat/ac4dec: Constify demuxer

The discrepancy between the definition and the declaration
in allformats.c is actually UB.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/ac4dec.c

@@ -93,7 +93,7 @@ static int ac4_read_packet(AVFormatContext *s, AVPacket *pkt)
     return ret;
 }
 
-AVInputFormat ff_ac4_demuxer = {
+const AVInputFormat ff_ac4_demuxer = {
     .name           = "ac4",
     .long_name      = NULL_IF_CONFIG_SMALL("raw AC-4"),
     .read_probe     = ac4_probe,