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/cpptools/stringtable.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef STRINGTABLE_H
-#define STRINGTABLE_H
+#pragma once
 
 #include <QAtomicInt>
 #include <QMutex>
@@ -69,5 +68,3 @@ private:
 
 } // Internal namespace
 } // CppTools namespace
-
-#endif // STRINGTABLE_H