networkmanager-openvpn: remove nm-gtk dependency

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

···
   nativeBuildInputs = [ intltool pkgconfig libxml2 ];
 
   configureFlags = [
+    "--without-libnm-glib"
     "--with-gnome=${if withGnome then "yes" else "no"}"
     "--localstatedir=/" # needed for the management socket under /run/NetworkManager
   ];