I420ToI400 fix for unused u and v arguments

TBR=kjellander@chromium.org
BUG=libyuv:679
TEST=None

Change-Id: I830f31e0de58c967cb02c71f422df4c19c94c367
Reviewed-on: https://chromium-review.googlesource.com/441949Reviewed-by: Frank Barchard <fbarchard@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>

source/planar_functions.cc

@@ -240,6 +240,10 @@ int I420ToI400(const uint8* src_y,
                int dst_stride_y,
                int width,
                int height) {
+  (void)src_u;
+  (void)src_stride_u;
+  (void)src_v;
+  (void)src_stride_v;
   if (!src_y || !dst_y || width <= 0 || height == 0) {
     return -1;
   }