pppd: fix build with libxcrypt

pkgs/tools/networking/ppp/default.nix

@@ -3,6 +3,7 @@
 , fetchFromGitHub
 , substituteAll
 , libpcap
+, libxcrypt
 , openssl
 , bash
 }:
@@ -31,6 +32,7 @@ stdenv.mkDerivation rec {
 
   buildInputs = [
     libpcap
+    libxcrypt
     openssl
     bash
   ];
@@ -48,6 +50,8 @@ stdenv.mkDerivation rec {
     "CC=${stdenv.cc.targetPrefix}cc"
   ];
 
+  NIX_LDFLAGS = "-lcrypt";
+
   installPhase = ''
     runHook preInstall
     mkdir -p $out/bin