avformat: Constify all muxer/demuxers

This is possible now that the next-API is gone.

Signed-off-by: Andreas Rheinhardt <[email protected]>
Signed-off-by: James Almer <[email protected]>
diff --git a/libavformat/c93.c b/libavformat/c93.c
index 256b980..9ecf142 100644
--- a/libavformat/c93.c
+++ b/libavformat/c93.c
@@ -185,7 +185,7 @@
     return 0;
 }
 
-AVInputFormat ff_c93_demuxer = {
+const AVInputFormat ff_c93_demuxer = {
     .name           = "c93",
     .long_name      = NULL_IF_CONFIG_SMALL("Interplay C93"),
     .priv_data_size = sizeof(C93DemuxContext),