openldap: fixup build on *-darwin

https://hydra.nixos.org/build/195641103

This essentially partially reverts dd331ad167.
I don't expect issues, as libxcrypt it meant to replace parts dropped
from glibc which's never been used on *-darwin anyway.

pkgs/development/libraries/openldap/default.nix

@@ -43,8 +43,8 @@ stdenv.mkDerivation rec {
     libsodium
     libtool
     openssl
-    libxcrypt
   ] ++ lib.optionals (stdenv.isLinux) [
+    libxcrypt # causes linking issues on *-darwin
     systemdMinimal
   ];