nixos/gnome3: add gnome-flashback to systemd.packages

When we did the revert of adding gnome-flashback to systemd.packages [0]
I forgot to test with other display managers. If we use GDM with gnome-flashback
it appears it doesn't try to fallback to non-systemd startup and always fails and
starts the regular gnome-session. So adding gnome-flashback to systemd.packages
was perfectly fine, but we did forgot one detail. We need systemd targets for the
customSessions which is added using  mkSystemdTargetForWm in the gnome-
flashback package.

[0]: 42f567b30d

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

@@ -184,6 +184,13 @@ in
         enableGnomeKeyring = true;
       };
 
+      systemd.packages = with pkgs.gnome3; [
+        gnome-flashback
+      ] ++ (map
+        (wm: gnome-flashback.mkSystemdTargetForWm {
+          inherit (wm) wmName;
+        }) cfg.flashback.customSessions);
+
       services.dbus.packages = [
         pkgs.gnome3.gnome-screensaver
       ];