List kali-linux-firmware from installation

Merged Arnaud Rebillout requested to merge add-firmware into master

So far, it was installed anyway, via the dependency chain:

kali-linux-default -> kali-linux-headless -> kali-linux-firmware

However in a near future we'll remove kali-linux-firmware from the dependencies of kali-linux-headless, so we want to list kali-linux-firmware for installation manually from now on.