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

benchmarks/CSSBenchmark.h

@@ -7,8 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
-#ifndef __CSS_BENCHMARK_H
-#define __CSS_BENCHMARK_H
+#pragma once
 
 #include <stdlib.h>
 #include <stdio.h>
@@ -40,5 +39,3 @@ void __printBenchmarkResult(char *name, clock_t start, clock_t end) {
   printf("%lf ms", mean);
   printf("\n");
 }
-
-#endif