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/externaltool.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef EXTERNALTOOL_H
-#define EXTERNALTOOL_H
+#pragma once
 
 #include <utils/fileutils.h>
 #include <utils/environment.h>
@@ -159,5 +158,3 @@ private:
 } // Core
 
 Q_DECLARE_METATYPE(Core::Internal::ExternalTool *)
-
-#endif // EXTERNALTOOL_H