[FIX] sale,purchase: Add currency symbol in tree view

In invisible to make the 'monetary' widget work.
Makes sense as it is the same behaviour as the form view where the currency is displayed on monetary fields, even if the use has no multicurrency acces.

Closes #14714

addons/purchase/views/purchase_views.xml

@@ -384,6 +384,7 @@
                     <field name="origin"/>
                     <field name="amount_untaxed" sum="Total Untaxed amount" string="Untaxed" widget="monetary"/>
                     <field name="amount_total" sum="Total amount" widget="monetary"/>
+                    <field name="currency_id" invisible="1"/>
                     <field name="state"/>
                     <field name="invoice_status" invisible="not context.get('show_purchase', False)"/>
                 </tree>

addons/sale/views/sale_views.xml

@@ -116,6 +116,7 @@
                     <field name="partner_id"/>
                     <field name="user_id"/>
                     <field name="amount_total" sum="Total Tax Included" widget="monetary"/>
+                    <field name="currency_id" invisible="1"/>
                     <field name="invoice_status"/>
                     <field name="state" invisible="1"/>
                 </tree>
@@ -134,6 +135,7 @@
                     <field name="partner_id"/>
                     <field name="user_id"/>
                     <field name="amount_total" sum="Total Tax Included" widget="monetary"/>
+                    <field name="currency_id" invisible="1"/>
                     <field name="state"/>
                 </tree>
             </field>