avcodec: fix some const warnings

libavcodec/options.c:583: warning: assignment discards qualifiers from pointer
target type
libavcodec/options.c:589: warning: initialization discards qualifiers from
pointer target type

libavcodec/options.c

@@ -580,13 +580,13 @@ int avcodec_get_context_defaults3(AVCodecContext *s, AVCodec *codec){
             }
         }
         if(codec->priv_class){
-            *(AVClass**)s->priv_data= codec->priv_class;
+            *(const AVClass**)s->priv_data = codec->priv_class;
             av_opt_set_defaults(s->priv_data);
         }
     }
     if (codec && codec->defaults) {
         int ret;
-        AVCodecDefault *d = codec->defaults;
+        const AVCodecDefault *d = codec->defaults;
         while (d->key) {
             ret = av_set_string3(s, d->key, d->value, 0, NULL);
             av_assert0(ret >= 0);