Nim: Switch to the now hopefully final PluginPrivate pattern

Change-Id: Ifffe56107974cfe9364b468dd3c3f5d69a9956d0
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/nim/nimplugin.h

@@ -46,6 +46,9 @@ private slots:
     void testNimParser_data();
     void testNimParser();
 #endif
+
+private:
+    class NimPluginPrivate *d = nullptr;
 };
 
-}
+} // Nim