nixos/gnome-desktop: fix adding printers with GNOME Control Center

...by adding system-config-printer to services.dbus.packages (if
services.printing.enable is true).

Without this patch, trying to add a printer will result in a little dialog
saying "Failed to add new printer" and gnome-control-center will print this to
the terminal (line wrapped):

(gnome-control-center:3546): printers-cc-panel-WARNING **: \
  GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: \
  The name org.fedoraproject.Config.Printing was not provided by any .service files

system-config-printer supplies the "org.fedoraproject.Config.Printing" dbus
service, thus fixing the problem.