lavfi/vf_thumbnail_cuda: fix operator precedence bug

Discovered via a warning when building with clang
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>

libavfilter/vf_thumbnail_cuda.cu

@@ -71,7 +71,7 @@ __global__ void Thumbnail_ushort2(cudaTextureObject_t ushort2_tex,
     {
         ushort2 pixel = tex2D<ushort2>(ushort2_tex, x, y);
         atomicAdd(&histogram[(pixel.x + 128) >> 8], 1);
-        atomicAdd(&histogram[256 + (pixel.y + 128) >> 8], 1);
+        atomicAdd(&histogram[256 + ((pixel.y + 128) >> 8)], 1);
     }
 }