[FIX] warning: fix override of product_id_change

Without the @api.multi property,
the method is considered as taking only one parameter (self?),
while it takes multiple parameters.

This prevented to change the quote template on a sale order,
if the quote template had some order lines, due to the below error

`TypeError: product_id_change() takes exactly 1 argument (19 given)`

opw-687797

addons/warning/warning.py

@@ -232,6 +232,7 @@ class sale_order_line(osv.osv):
             result['warning'] = warning
         return result
 
+    @api.multi
     @api.onchange('product_id')
     def product_id_change(self):
         warning = self.onchange_product_id_warning()