Fix #110497: Quick favorites can be duplicated

Mistake in adb370e6ba
Skip "add to quick favorites" button from context menu when button is
found in user_menu.

Pull Request: https://projects.blender.org/blender/blender/pulls/110517

source/blender/editors/interface/interface_context_menu.cc

@@ -1030,6 +1030,7 @@ bool ui_popup_context_menu_for_button(bContext *C, uiBut *but, const wmEvent *ev
             0,
             0,
             "");
+        item_found = true;
         UI_but_func_set(but2, [um, umi](bContext &) {
           U.runtime.is_dirty = true;
           ED_screen_user_menu_item_remove(&um->items, umi);