use #pragma once

Summary: #pragma once is widely supported and is a lot harder to get wrong than #ifdef include guards.

Reviewed By: lucasr

Differential Revision: D3648895

fbshipit-source-id: faf42cda82764adaf41cf3f3f9109d48aea203fe

CSSLayout/CSSLayout-internal.h

@@ -7,8 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
-#ifndef __CSS_LAYOUT_INTERNAL_H
-#define __CSS_LAYOUT_INTERNAL_H
+#pragma once
 
 #include "CSSLayout.h"
 #include "CSSNodeList.h"
@@ -98,5 +97,3 @@ typedef struct CSSNode {
 } CSSNode;
 
 CSS_EXTERN_C_END
-
-#endif