[FIX] point_of_sale: missing variable declaration

opw-2169164

closes odoo/odoo#43073

X-original-commit: 5432e24a990885a68df5a691388eb5c3584d9759
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/point_of_sale/static/src/js/models.js

@@ -1948,7 +1948,7 @@ exports.Orderline = Backbone.Model.extend({
                 else if(tax.amount_type === 'fixed')
                     incl_fixed_amount += quantity * tax.amount
                 else{
-                    tax_amount = self._compute_all(tax, base, quantity);
+                    var tax_amount = self._compute_all(tax, base, quantity);
                     incl_fixed_amount += tax_amount;
                     cached_tax_amounts[i] = tax_amount;
                 }