[FIX] pos_restaurant: use odoo groupby instead itertools

closes odoo/odoo#105518

Signed-off-by: Olivier Dony (odo) <odo@odoo.com>

addons/pos_restaurant/models/pos_order.py

 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
-from itertools import groupby
+from odoo.tools import groupby
 from re import search
 from functools import partial