[FIX] account: tax sign on vendor bill tree

On the 'Vendors > Refund' view, the fields 'Tax Excluded' and 'Tax' are
positive, while the 'Total' is negative.

We apply the same loginc than d7c0c5c1.

opw-2115019

closes odoo/odoo#39652

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>