Recognize wxAUI_NB_MIDDLE_CLICK_CLOSE in wxAuiNotebook XRC handler

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

See #17597.

(cherry picked from commit 63944ae9bd)

src/xrc/xh_auinotbk.cpp

@@ -35,6 +35,7 @@ wxAuiNotebookXmlHandler::wxAuiNotebookXmlHandler()
     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);