networkmanager-fortisslvpn: remove nm-gtk dependency

pkgs/tools/networking/network-manager/fortisslvpn/default.nix

···
   nativeBuildInputs = [ intltool pkgconfig ];
 
   configureFlags = [
+    "--without-libnm-glib"
     "--with-gnome=${if withGnome then "yes" else "no"}"
     "--localstatedir=/tmp"
   ];