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/classview/classviewconstants.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef CLASSVIEWCONSTANTS_H
-#define CLASSVIEWCONSTANTS_H
+#pragma once
 
 namespace ClassView {
 namespace Constants {
@@ -40,5 +39,3 @@ enum ItemRole
 
 } // namespace Constants
 } // namespace ClassView
-
-#endif // CLASSVIEWCONSTANTS_H