re-add __clone

I re-added the `__clone` method which removed in the last commit.

Creational/Singleton/Singleton.php

@@ -28,6 +28,13 @@ final class Singleton
     {
     }
 
+    /**
+     * prevent the instance from being cloned (which would create a second instance of it)
+     */
+    private function __clone()
+    {
+    }
+    
     /**
      * prevent from being unserialized (which would create a second instance of it)
      */