otpw: fix build with libxcrypt

pkgs/os-specific/linux/otpw/default.nix

···
-{ lib, stdenv, fetchurl, pam }:
+{ lib, stdenv, fetchurl, pam, libxcrypt }:
 
 stdenv.mkDerivation rec {
   pname = "otpw";
···
     cp *.8 $out/share/man/man8
   '';
 
-  buildInputs = [ pam ];
+  buildInputs = [ pam libxcrypt ];
 
   hardeningDisable = [ "stackprotector" ];