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/dss.c b/libavformat/dss.c
index 463ecd9..ee3a099 100644
--- a/libavformat/dss.c
+++ b/libavformat/dss.c
@@ -354,7 +354,7 @@
 }
 
 
-AVInputFormat ff_dss_demuxer = {
+const AVInputFormat ff_dss_demuxer = {
     .name           = "dss",
     .long_name      = NULL_IF_CONFIG_SMALL("Digital Speech Standard (DSS)"),
     .priv_data_size = sizeof(DSSDemuxContext),