libavcodec: Mark AVCodec->priv_class const

This avoids warnings of this kind, everywhere priv_class
is initialized:

warning: initialization discards qualifiers from pointer target type
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/avcodec.h

@@ -2944,7 +2944,7 @@ typedef struct AVCodec {
     const enum AVSampleFormat *sample_fmts; ///< array of supported sample formats, or NULL if unknown, array is terminated by -1
     const int64_t *channel_layouts;         ///< array of support channel layouts, or NULL if unknown. array is terminated by 0
     uint8_t max_lowres;                     ///< maximum value for lowres supported by the decoder
-    AVClass *priv_class;                    ///< AVClass for the private context
+    const AVClass *priv_class;              ///< AVClass for the private context
     const AVProfile *profiles;              ///< array of recognized profiles, or NULL if unknown, array is terminated by {FF_PROFILE_UNKNOWN}
 
     /**