Wholesale conversion to #pragma once

Kudos to cgmb and https://github.com/cgmb/guardonce

Change-Id: Ifa8970734b8d43fd08c9260c645bdb0228633791
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/coreplugin/messagebox.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef MESSAGEBOX_H
-#define MESSAGEBOX_H
+#pragma once
 
 #include "core_global.h"
 
@@ -40,6 +39,3 @@ namespace AsynchronousMessageBox {
     CORE_EXPORT void critical(const QString &title, const QString &desciption);
 }
 }
-
-#endif // MESSAGEBOX_H
-