fix: some code tidy about includes & irr namespaces

src/client/client.cpp

@@ -1475,7 +1475,7 @@ bool Client::updateWieldedItem()
 	return true;
 }
 
-irr::scene::ISceneManager* Client::getSceneManager()
+scene::ISceneManager* Client::getSceneManager()
 {
 	return m_rendering_engine->get_scene_manager();
 }