Added missing include to os2.cc. 

os2.cc uses stddef.h's offsetof().

Original review: http://codereview.chromium.org/7113013/

git-svn-id: http://ots.googlecode.com/svn/trunk@69 a4e77c2c-9104-11de-800e-5b313e0d2bf3
diff --git a/src/ots.h b/src/ots.h
index 09f3434..c1741e2 100644
--- a/src/ots.h
+++ b/src/ots.h
@@ -5,6 +5,7 @@
 #ifndef OTS_H_
 #define OTS_H_
 
+#include <stddef.h>
 #include <cstdarg>
 #include <cstddef>
 #include <cstdio>