[FIX] base: add default_order in pivot view

The option is supposed to be supported:
https://github.com/odoo/odoo/blob/3316fd49247d14dd2ab39c1267ed940feafdc664/addons/web/static/src/js/views/pivot/pivot_model.js#L363



Closes #39692
opw-2169539

closes odoo/odoo#43255

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

odoo/addons/base/rng/pivot_view.rng

@@ -12,6 +12,7 @@
             <rng:optional><rng:attribute name="stacked"/></rng:optional>
             <rng:optional><rng:attribute name="display_quantity"/></rng:optional>
             <rng:optional><rng:attribute name="disable_linking"/></rng:optional>
+            <rng:optional><rng:attribute name="default_order"/></rng:optional>
             <rng:zeroOrMore>
                 <rng:ref name="field"/>
             </rng:zeroOrMore>