[FIX] purchase : Missing access right for User in Purchase group

Issue: When closing a PoS session, the user can Cash In/Out of the
register but it requires Accounting/Billing group or Purchase/User
group. However with Purchase/User group, there was an access right
error (not allowed to create account.move.line

Steps to reproduce :
 1) Install PoS Module
 2) PoS > Your Shop > Settings and enable Advanced Cash Control
 3) As administrator and in debug mode go to
  Settings > Users & Companies > Users
 4) Remove Accounting, Inventory and Purchase group of [User A]
 5) Log in with [User A], open a PoS Session, close it, and in Cash
 Control form, click Cash In/Out
 6) Remove or add money
 -> Access right error, you need to be Accounting/Billing or
 Purchase/User
 7) As administrator, add the User group to Purchase for [User A]
 8) As [User A], click Cash In/Out, removed or add money
 -> Still access right error

Why is that a bug:
 The first warning asks us to give the Purchase/User group to the user
 if we want to Cash In/Out but even when doing that, the access rights
 are still not enough to allow the creation of the entry

opw-2611298

closes odoo/odoo#75029

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>