avcodec: Constify AVCodecs

Given that the AVCodec.next pointer has now been removed, most of the
AVCodecs are not modified at all any more and can therefore be made
const (as this patch does); the only exceptions are the very few codecs
for external libraries that have a init_static_data callback.

Signed-off-by: Andreas Rheinhardt <[email protected]>
Signed-off-by: James Almer <[email protected]>
diff --git a/libavcodec/subviewerdec.c b/libavcodec/subviewerdec.c
index 805c7dd..35c1ad7 100644
--- a/libavcodec/subviewerdec.c
+++ b/libavcodec/subviewerdec.c
@@ -65,7 +65,7 @@
     return avpkt->size;
 }
 
-AVCodec ff_subviewer_decoder = {
+const AVCodec ff_subviewer_decoder = {
     .name           = "subviewer",
     .long_name      = NULL_IF_CONFIG_SMALL("SubViewer subtitle"),
     .type           = AVMEDIA_TYPE_SUBTITLE,