nixos/gvfs: fix libmtp udev package path for realz

bin is the primary output so the previous attempt at fixing this
(https://github.com/NixOS/nixpkgs/commit/2d7fc66c79d7543ada565c96a3c6ad2b15c6b350)
was a no-op.

nixos/modules/services/desktops/gvfs.nix

@@ -54,7 +54,7 @@ in
 
     systemd.packages = [ cfg.package ];
 
-    services.udev.packages = [ pkgs.libmtp ];
+    services.udev.packages = [ pkgs.libmtp.out ];
 
     # Needed for unwrapped applications
     environment.variables.GIO_EXTRA_MODULES = [ "${cfg.package}/lib/gio/modules" ];