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.

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -119,6 +119,7 @@ in {
     services.telepathy.enable = mkDefault true;
     networking.networkmanager.enable = mkDefault true;
     services.upower.enable = config.powerManagement.enable;
+    services.dbus.packages = mkIf config.services.printing.enable [ pkgs.system-config-printer ];
     hardware.bluetooth.enable = mkDefault true;
 
     fonts.fonts = [ pkgs.dejavu_fonts pkgs.cantarell_fonts ];