nixos/gnome3: fix screen sharing

Without this change there will be silent errors when enabling screen
sharing. The GUI thinks it enables the service when it in fact does not
(errors are seen in the system journal).

vino is already in the closure of gnome-control-center, so this is
basically free.

Configuration of screen sharing is done in GNOME control center.

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

@@ -124,6 +124,7 @@ in {
     hardware.bluetooth.enable = mkDefault true;
     services.xserver.libinput.enable = mkDefault true; # for controlling touchpad settings via gnome control center
     services.udev.packages = [ pkgs.gnome3.gnome_settings_daemon ];
+    systemd.packages = [ pkgs.gnome3.vino ];
 
     # If gnome3 is installed, build vim for gtk3 too.
     nixpkgs.config.vim.gui = "gtk3";