fftools/cmdutils: constify the first parameter of filter_codec_opts()

fftools/cmdutils.c

@@ -943,7 +943,7 @@ int check_stream_specifier(AVFormatContext *s, AVStream *st, const char *spec)
     return ret;
 }
 
-AVDictionary *filter_codec_opts(AVDictionary *opts, enum AVCodecID codec_id,
+AVDictionary *filter_codec_opts(const AVDictionary *opts, enum AVCodecID codec_id,
                                 AVFormatContext *s, AVStream *st, const AVCodec *codec)
 {
     AVDictionary    *ret = NULL;

fftools/cmdutils.h

@@ -335,7 +335,7 @@ int check_stream_specifier(AVFormatContext *s, AVStream *st, const char *spec);
  *              If null, the default one is looked up according to the codec id.
  * @return a pointer to the created dictionary
  */
-AVDictionary *filter_codec_opts(AVDictionary *opts, enum AVCodecID codec_id,
+AVDictionary *filter_codec_opts(const AVDictionary *opts, enum AVCodecID codec_id,
                                 AVFormatContext *s, AVStream *st, const AVCodec *codec);
 
 /**