Cleanup cxxreact BUCK file

Reviewed By: mzlee

Differential Revision: D5020563

fbshipit-source-id: 286a2d1c44623b5b9fd923ef8684d263500791b3

ReactCommon/cxxreact/SampleCxxModule.h

@@ -1,13 +1,12 @@
 // Copyright 2004-present Facebook. All Rights Reserved.
 
-#ifndef FBSAMPLEXPLATMODULE
-#define FBSAMPLEXPLATMODULE
-
-#include <cxxreact/CxxModule.h>
+#pragma once
 
 #include <memory>
 #include <vector>
 
+#include <cxxreact/CxxModule.h>
+
 namespace facebook { namespace xplat { namespace samples {
 
 // In a less contrived example, Sample would be part of a traditional
@@ -48,5 +47,4 @@ private:
 
 }}}
 
-#endif
-
+extern "C" facebook::xplat::module::CxxModule *SampleCxxModule();