networkmanager-vpnc: remove nm-gtk dependency

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

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