services: WPA Supplicant: Conditionally depend on D-Bus.

Fixes <https://bugs.gnu.org/43567>.
Reported by calcium <calcium@disroot.org>.

* gnu/services/networking.scm (<wpa-supplicant-configuration>)[requirement]:
Remove 'dbus-system.
(wpa-supplicant-shepherd-service)[requirement]: Add 'dbus-system when DBUS? is true.
* doc/guix.texi (Networking Services)[wpa-supplicant-service-type]: Adjust
accordingly.