Fix type of out[] variable, it should not be const.

Fixes compiler warning about incompatible types in sws_scale call.

libavfilter/vf_scale.c

@@ -270,7 +270,8 @@ static int scale_slice(AVFilterLink *link, struct SwsContext *sws, int y, int h,
     ScaleContext *scale = link->dst->priv;
     AVFilterBufferRef *cur_pic = link->cur_buf;
     AVFilterBufferRef *out_buf = link->dst->outputs[0]->out_buf;
-    const uint8_t *in[4], *out[4];
+    const uint8_t *in[4];
+    uint8_t *out[4];
     int in_stride[4],out_stride[4];
     int i;