The answer is pretty simple when you use .xcassets (Assets). What you have to do is to select your AppIcon collection and on the right pane (Inspector) select Pre-rendered icon. and that's it.
- In the Project Navigator, Click on project name
- In the App Icons section, press Use Asset Catalog
- Click import
- Click on the small arrow next to the combo box
In the Attribute Inspector, check iOS icon is pre-rendered