[FIX] stock: @api.model on _prepare_values_extra_move

In saas-11 stock was migrated to new API, but some old API still called
_prepare_values_extra_move so it needs to be callable via both API
version.

opw-687958

addons/stock/models/stock_picking.py

@@ -911,6 +911,7 @@ class Picking(models.Model):
             moves.action_confirm()
         return moves
 
+    @api.model
     def _prepare_values_extra_move(self, op, product, remaining_qty):
         """
         Creates an extra move when there is no corresponding original move to be copied