Decal is not an object, but rather a helper/mapping projector, therefore it can't be used in lights include/exclude list. If you really need to use separate lighting for decal, consider replacing it with floating plane object with opacity. You can use it in combination with rayswitch material, so that it does not cast shadow on the base object. This technique can be somewhat tricky to use if your base object is not flat, or it has pronounced displacement, but even in such case it's not impossible.