Wizards: Support using #pragma once instead of include guards

Allow users to choose #pragma once instead of #ifndef include guards in
generated header files.

Fixes: QTCREATORBUG-12166
Change-Id: I3ba41c7570beb9c5958e174b5581fcc25855050f
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Filip Bucek <fbucek@atlas.cz>

src/plugins/cpptools/cpptoolsplugin.h

@@ -65,6 +65,7 @@ public:
     static void clearHeaderSourceCache();
     static Utils::FileName licenseTemplatePath();
     static QString licenseTemplate();
+    static bool usePragmaOnce();
 
     bool initialize(const QStringList &arguments, QString *errorMessage) final;
     void extensionsInitialized() final;