GenericProject: Use standard layout for IPlugin subclasses

Change-Id: I946b65081ebeab10afeb39a76d05e66f04b2ebac
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/genericprojectmanager/genericprojectplugin.h

@@ -41,6 +41,8 @@ public:
 private:
     bool initialize(const QStringList &arguments, QString *errorString) override;
     void extensionsInitialized() override { }
+
+    class GenericProjectPluginPrivate *d = nullptr;
 };
 
 } // namespace Internal