Allow building Kushite mercenary camps in neutral territory.

This gives Kushites more interesting gameplay options, and is consistant with other mercenary camps.
Kushite mercenaries lived outside of the core Kushite territory, thus giving a historical ground to this change.

Accepted by: ValihrAnt, borg-, scythetwirler, Angen, badosu, sundiata

Patch by: Nescio

Differential Revision: https://code.wildfiregames.com/D2534