[FIX] base: missing rng attribute 'force_save' on fields

Th view validation when attribute force_save is set on a field raise an
'Invalidation error' because there is no rng attribute for 'force_save'.

This attribute disappeared during the refactoring in rev: https://github.com/odoo/odoo/commit/70942e4cfb7a8993904b4d142e3b1749a40db806

odoo/addons/base/rng/common.rng

@@ -261,6 +261,14 @@
             <rng:optional><rng:attribute name="avatar_field" /></rng:optional>
             <rng:optional><rng:attribute name="write_model" /></rng:optional>
             <rng:optional><rng:attribute name="write_field" /></rng:optional>
+            <rng:optional>
+                <rng:attribute name="force_save">
+                    <rng:choice>
+                      <rng:value>1</rng:value>
+                      <rng:value>0</rng:value>
+                    </rng:choice>
+                </rng:attribute>
+            </rng:optional>
             <!-- Widget *static* options defined as an arbitrary JSON dict, with
                  widget-dependent parameters. To be ignored if widget/client does
                  not support them. -->