vf_copy: silence "incompatible pointer type" warning

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

libavfilter/vf_copy.c

@@ -37,7 +37,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         return AVERROR(ENOMEM);
     }
     av_frame_copy_props(out, in);
-    av_image_copy(out->data, out->linesize, in->data, in->linesize,
+    av_image_copy(out->data, out->linesize, (const uint8_t**) in->data, in->linesize,
                   in->format, in->width, in->height);
 
     av_frame_free(&in);