[FIX] point_of_sale: Add internal product reference to invoice

Current behavior:
The internal product reference is not added to the invoice when the
invoice is created from the POS.

Steps to reproduce:
- Create an order in the POS
- Validate the order and print the invoice
- Open the invoice, the internal product reference is not written on the
  invoice

opw-3119678

closes odoo/odoo#111371

Signed-off-by: Engels Robin (roen) <roen@odoo.com>