aacenc: Make chan_map const correct

libavcodec/aacenc.h

@@ -61,7 +61,7 @@ typedef struct AACEncContext {
     int16_t *samples;                            ///< saved preprocessed input
 
     int samplerate_index;                        ///< MPEG-4 samplerate index
-    uint8_t *chan_map;                           ///< channel configuration map
+    const uint8_t *chan_map;                     ///< channel configuration map
 
     ChannelElement *cpe;                         ///< channel elements
     FFPsyContext psy;