pkg-stage.sh: add x11/sddm

Approved by: re (cperciva)
MFC After: 2 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/996

(cherry picked from commit 5ff76402daf17b9c36048bf11957dac7556272a6)
(cherry picked from commit 9317a4a3a82797f8128720ac46a36ece0a404349)