pure-ftpd: fix build with libxcrypt

pkgs/servers/ftp/pure-ftpd/default.nix

···
-{ lib, stdenv, fetchurl, openssl, pam, fetchpatch }:
+{ lib, stdenv, fetchurl, openssl, pam, libxcrypt }:
 
 stdenv.mkDerivation rec {
   pname = "pure-ftpd";
···
     sha256 = "sha256-QWD2a3ZhXuojl+rE6j8KFGt5KCB7ebxMwvma17e9lRM=";
   };
 
-  buildInputs = [ openssl pam ];
+  buildInputs = [ openssl pam libxcrypt ];
 
   configureFlags = [ "--with-tls" ];