avcodec: Constify all the AVCodecParsers

Possible now that the next pointer no longer exists.

Signed-off-by: Andreas Rheinhardt <[email protected]>
Signed-off-by: James Almer <[email protected]>
diff --git a/libavcodec/h261_parser.c b/libavcodec/h261_parser.c
index 2299c1c..e0b84c5 100644
--- a/libavcodec/h261_parser.c
+++ b/libavcodec/h261_parser.c
@@ -86,7 +86,7 @@
     return next;
 }
 
-AVCodecParser ff_h261_parser = {
+const AVCodecParser ff_h261_parser = {
     .codec_ids      = { AV_CODEC_ID_H261 },
     .priv_data_size = sizeof(ParseContext),
     .parser_parse   = h261_parse,