Add ability to use pppd to xl2tpd

pkgs/tools/networking/xl2tpd/default.nix

···
-{ stdenv, fetchFromGitHub, libpcap }:
+{ stdenv, fetchFromGitHub, libpcap, ppp }:
 
 let version = "1.3.6";
 in stdenv.mkDerivation {
···
   };
 
   buildInputs = [ libpcap ];
+
+  postPatch = ''
+    substituteInPlace l2tp.h --replace /usr/sbin/pppd ${ppp}/sbin/pppd
+  '';
 
   makeFlags = [ "PREFIX=$(out)" ];