nntp-proxy: fix build with libxcrypt

pkgs/applications/networking/nntp-proxy/default.nix

···
-{ lib, stdenv, fetchFromGitHub, libconfig, pkg-config, libevent, openssl }:
+{ lib, stdenv, fetchFromGitHub, libconfig, pkg-config, libevent, openssl, libxcrypt }:
 
 stdenv.mkDerivation {
   pname = "nntp-proxy";
···
   };
 
   nativeBuildInputs = [ pkg-config ];
-  buildInputs = [ libconfig libevent openssl ];
+  buildInputs = [ libconfig libevent openssl libxcrypt ];
 
   installFlags = [ "INSTALL_DIR=$(out)/bin/" ];