Add child_qty => stock_qty

[bupp925]

https://github.com/SearchSpring/magento2-searchspring-feed/blob/master/Helper/Generator.php#L280

The existing "stock_qty" field is empty for Configurable Items. I understand how the saleable field works, but it'd still be much better to have an inventory number in this "stock_qty" field as well - because it's only a matter of time before we need it someday: "Only 1 Left in Stock", Boost by Stock Count, etc ...