gnome-extra/gnome-user-share: add missing systemd build req

gnome-extra/gnome-user-share: add missing systemd build req

Theoretically gnome-user-share works without systemd, but it has
an unnecessary check for systemd in its configure check. Add that
missing requirement.
However while it could be manually launched without systemd, the
integration support in gnome-settings-daemon/gnome-control-center
will try to launch it via systemd specific dbus interface, thus
users would have to handle it themselves for now.
Can be reconsidered on next bump to support it, but output
intructions what to do without systemd, or maybe even some support
code in settings-daemon. Help welcome.

Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>