gnome3.gdm: add upstream xsession

nixos/modules/services/x11/display-managers/gdm.nix

@@ -173,6 +173,8 @@ in
       ${optionalString cfg.gdm.debug "Enable=true"}
     '';
 
+    environment.etc."gdm/Xsession".source = "${pkgs.gnome3.gdm}/etc/gdm/Xsession";
+
     # GDM LFS PAM modules, adapted somehow to NixOS
     security.pam.services = {
       gdm-launch-environment.text = ''

pkgs/desktops/gnome-3/core/gdm/default.nix

@@ -21,6 +21,7 @@ stdenv.mkDerivation rec {
     "--sysconfdir=/etc"
     "--localstatedir=/var"
     "--with-plymouth=yes"
+    "--enable-gdm-xsession"
     "--with-initial-vt=7"
     "--with-systemdsystemunitdir=$(out)/etc/systemd/system"
   ];