[x11-misc/sddm] Add user/group creation

Upstream introduced support for running SDDM as non-root user in
https://github.com/sddm/sddm/commit/484395d23f2cdbde9df5d1efb25d5d2751b43aa9

Package-Manager: portage-2.2.10
Merge pull request #50 from eliasp/sddm-user-group

[x11-misc/sddm] Add user/group creation