HLE/SRV: Implemented RegisterService.

Now system modules can do more than just crash immediately on startup.

src/core/hle/service/sm/srv.h

@@ -28,6 +28,7 @@ private:
     void Subscribe(Kernel::HLERequestContext& ctx);
     void Unsubscribe(Kernel::HLERequestContext& ctx);
     void PublishToSubscriber(Kernel::HLERequestContext& ctx);
+    void RegisterService(Kernel::HLERequestContext& ctx);
 
     std::shared_ptr<ServiceManager> service_manager;
     Kernel::SharedPtr<Kernel::Semaphore> notification_semaphore;