[FIX] stock_account: costing method on product category

The field "costing method" on the product.category only appears when
the inventory valuation is set to "perpetual (automated)" which is
wrong. The user must be able to choose the costing method no matter
which inventory valuation method is chosen.

The problem is due to this commit b6a2c0a4
that changes the xml view and make the field invisible if the inventory
valuation is set to "Periodic (manual)".

This commit revert the change on the xml view and thus add again
the group attribute that should not have been removed.

addons/stock_account/views/product_views.xml

@@ -18,7 +18,7 @@
                 <group name="account_property" position="before">
                     <group>
                         <group string="Inventory Valuation">
-                            <field name="property_cost_method" attrs="{'invisible': [('property_valuation', '=', 'manual_periodic')]}"/>
+                            <field name="property_cost_method" groups="stock_account.group_inventory_valuation"/>
                             <field name="property_valuation"/>
                         </group>
                     </group>