[FIX] account_analytic_plans: creating an analytic distribution

When creating an analytic distribution, the field "plan_id" must be required.

opw:646788

addons/account_analytic_plans/account_analytic_plans_view.xml

@@ -74,7 +74,7 @@
                 <group col="4">
                     <field name="name"/>
                     <field name="code"/>
-                    <field name="plan_id"/>
+                    <field name="plan_id" required="1"/>
                     <field name="journal_id"/>
                 </group>
                 <field name="account_ids">