[FIX] import_xml: Error when using uid attribute on function tag

When defining a function tag containing the uid attribute, the following error is raised:
`AssertionError: Element odoo has extra content`

That attribute is parsed correctly on python, but it's missing from the RELAX NG validator.

This commit adds the missing entry to the validator.

closes odoo/odoo#49810

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

odoo/import_xml.rng

@@ -40,6 +40,7 @@
             <!-- doesn't look like it has any use? -->
             <rng:optional><rng:attribute name="id" /></rng:optional>
             <rng:optional><rng:attribute name="context" /></rng:optional>
+            <rng:optional><rng:attribute name="uid" /></rng:optional>
             <rng:optional>
                 <rng:choice>
                     <rng:attribute name="eval" />