gnome3.at-spi2-core: fix service not found error

The service was not registered as a systemd service resulting in errors
in the system journal every time a GNOME application was launched.

See: #16327

nixos/modules/services/desktops/gnome3/at-spi2-core.nix

@@ -34,6 +34,8 @@ with lib;
 
     services.dbus.packages = [ pkgs.at_spi2_core ];
 
+    systemd.packages = [ pkgs.at_spi2_core ];
+
   };
 
 }