cavs: fix "warning: initialization discards const qualifier from pointer target type"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cavs.c

@@ -276,7 +276,7 @@ static void intra_pred_plane(uint8_t *d,uint8_t *top,uint8_t *left,int stride)
     int x, y, ia;
     int ih = 0;
     int iv = 0;
-    uint8_t *cm = ff_cropTbl + MAX_NEG_CROP;
+    const uint8_t *cm = ff_cropTbl + MAX_NEG_CROP;
 
     for (x = 0; x < 4; x++) {
         ih += (x + 1) * (top [5 + x] - top [3 - x]);