AK: Bring Singleton into the global namespace with using

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/eb60cb156da

AK/Singleton.h

@@ -110,4 +110,7 @@ public:
 private:
     mutable Atomic<T*> m_obj { nullptr };
 };
+
 }
+
+using AK::Singleton;