avcodec/aacenc: mark output as const as its not written to

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aacenc.c

@@ -154,7 +154,7 @@ static void apply_window_and_mdct(AACEncContext *s, SingleChannelElement *sce,
                                   float *audio)
 {
     int i;
-    float *output = sce->ret_buf;
+    const float *output = sce->ret_buf;
 
     apply_window[sce->ics.window_sequence[0]](s->fdsp, sce, audio);