Recognize wxAUI_NB_MIDDLE_CLICK_CLOSE in wxAuiNotebook XRC handler

This style was apparently just forgotten, so add it too.

Closes #17597.

src/xrc/xh_aui.cpp

@@ -51,6 +51,7 @@ wxAuiXmlHandler::wxAuiXmlHandler()
     XRC_ADD_STYLE(wxAUI_NB_CLOSE_BUTTON);
     XRC_ADD_STYLE(wxAUI_NB_CLOSE_ON_ACTIVE_TAB);
     XRC_ADD_STYLE(wxAUI_NB_CLOSE_ON_ALL_TABS);
+    XRC_ADD_STYLE(wxAUI_NB_MIDDLE_CLICK_CLOSE);
     XRC_ADD_STYLE(wxAUI_NB_TOP);
     XRC_ADD_STYLE(wxAUI_NB_BOTTOM);