displaymanager: Run "sddm --example-config > /etc/sddm.conf" in a shell.

Otherwise, the output redirection cannot work (and the argument was also
misinterpreted as part of the command name).
Merge pull request #139 from kkofler/fix-sddm-chroot-call

displaymanager: Run "sddm --example-config > /etc/sddm.conf" in a shell.