Unlike gdm-service-type, sddm-service-type cannot be configured in an
operating-system configuration by simply adding (service
sddm-service-type). This adds the default SDDM configuration just like
GDM does..
* gnu/services/sddm.scm (sddm-service-type)[default-value]: New field.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>