https://bugzilla.mozilla.org/show_bug.cgi?id=628252

diff --git a/gfx/ots/src/os2.cc b/gfx/ots/src/os2.cc
--- a/gfx/ots/src/os2.cc
+++ b/gfx/ots/src/os2.cc
@@ -3,4 +3,6 @@
 // found in the LICENSE file.
 
+#include <stddef.h>
+
 #include "os2.h"