[FIX] mrp: attrs lot_id required in produce wizard

Commit b2cf955e made the lot_id always required in the
produce wizard of the manufacturing order (without work orders).

It should only be required when the finished product is tracked by lot.

addons/mrp/wizard/mrp_product_produce_views.xml

@@ -17,7 +17,7 @@
                                 <field name="product_uom_id" readonly="1"/>
                             </div>
                             <field name="product_tracking" invisible="1"/>
-                            <field name="lot_id" required="1" attrs="{'invisible': [('product_tracking', '=', 'none')]}" domain="[('product_id','=',product_id)]" context="{'default_product_id': product_id}"/>
+                            <field name="lot_id" attrs="{'invisible': [('product_tracking', '=', 'none')], 'required': [('product_tracking', '!=', 'none')]}" domain="[('product_id','=',product_id)]" context="{'default_product_id': product_id}"/>
                         </group>
                     </group>
                     <group>