[FIX] stock: add index

Before this commit, in large database, openning a sale.order can take 3s.
Because in module sale_purchase_stock, _get_purchase_order use a On2many field : stock_move_ids, with inverse field group_id.

Now it takes 200 ms.

closes odoo/odoo#110128

Signed-off-by: William Henrotin (whe) <whe@odoo.com>