nixos/gnome: enable MTP support in gvfs

To support browsing files on Android phones in Nautilus (and other GVFS
based file browsers).

nixos/modules/services/desktops/gnome3/gvfs.nix

@@ -1,6 +1,6 @@
 # gvfs backends
 
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
@@ -37,6 +37,8 @@ in
 
     services.dbus.packages = [ gnome3.gvfs ];
 
+    services.udev.packages = [ pkgs.libmtp ];
+
   };
 
 }