replace include guards with pragma once

src/util/platform.hpp

@@ -1,5 +1,4 @@
-#ifndef UTIL_PLATFORM_HPP_
-#define UTIL_PLATFORM_HPP_
+#pragma once
 
 #include <string>
 
@@ -8,5 +7,3 @@ namespace platform {
     // @return environment locale in ISO format ll_CC
     std::string detect_locale();
 }
-
-#endif  // UTIL_PLATFORM_HPP_