build: treat systemd-devel as a optional build dependency
Some systems (Alpine/postmarketOS) don't use systemd and only provide a alternative init manager, like OpenRC. With this change, also these systems can package and use gnome-user-share.
Some systems (Alpine/postmarketOS) don't use systemd and only provide a alternative init manager, like OpenRC. With this change, also these systems can package and use gnome-user-share.