--- a/mozilla/gfx/ots/src/os2.cc
+++ b/mozilla/gfx/ots/src/os2.cc
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <cstddef>
+
 #include "os2.h"
 
 #include "head.h"