nixos/gnome3: fix gnome-flashback

This session would fail to start because we didn't have it in systemd.packages
(as we've switched to systemd gnome-session).

Haven't tested custom sessions.

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

@@ -176,6 +176,10 @@ in
               wmCommand = "${pkgs.gnome3.metacity}/bin/metacity";
             } ++ cfg.flashback.customSessions);
 
+      systemd.packages = with pkgs.gnome3; [
+        gnome-flashback
+      ];
+
       security.pam.services.gnome-screensaver = {
         enableGnomeKeyring = true;
       };