Fix shiboken error

[hellantos]

When running rqt with PySide2 it does not shutdown cleanly.

PluginHandler.shutdown_plugin() plugin "rqt_topic/TopicPlugin#1" raised an exception:
Traceback (most recent call last):
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/plugin_handler.py", line 136, in shutdown_plugin
    self._shutdown_plugin()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/plugin_handler_direct.py", line 92, in _shutdown_plugin
    self.emit_shutdown_plugin_completed()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/plugin_handler.py", line 150, in emit_shutdown_plugin_completed
    callback(self._instance_id)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/plugin_manager.py", line 480, in _close_application_shutdown_callback
    self._close_application_signal()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/plugin_manager.py", line 483, in _close_application_signal
    self._plugin_provider.shutdown()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/composite_plugin_provider.py", line 87, in shutdown
    plugin_provider.shutdown()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/composite_plugin_provider.py", line 87, in shutdown
    plugin_provider.shutdown()
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/qt_gui/composite_plugin_provider.py", line 87, in shutdown
    plugin_provider.shutdown()
  File "/home/christoph/ws_ros2/install/qt_gui_cpp/local/lib/python3.10/dist-packages/qt_gui_cpp/ros_pluginlib_plugin_provider.py", line 95, in shutdown
    self._plugin_provider.shutdown()
AttributeError: 'libqt_gui_cpp_shiboken.qt_gui_cpp.RosPluginlibPlug' object has no attribute 'shutdown

Shiboken2 does not generate or optimizes the shutdown function away. This fix adds it manually - it is an empty function anyway.