knot-dns: another attempt to fix build on Darwin

pkgs/servers/dns/knot-dns/default.nix

@@ -1,5 +1,6 @@
 { stdenv, fetchurl, pkgconfig, gnutls, jansson, liburcu, lmdb, libcap_ng, libidn
-, systemd, nettle, libedit }:
+, systemd, nettle, libedit, zlib
+}:
 
 # Note: ATM only the libraries have been tested in nixpkgs.
 stdenv.mkDerivation rec {
@@ -20,7 +21,8 @@ stdenv.mkDerivation rec {
     # without sphinx &al. for developer documentation
   ]
     # Use embedded lmdb there for now, as detection is broken on Darwin somehow.
-    ++ stdenv.lib.optionals stdenv.isLinux [ libcap_ng systemd lmdb ];
+    ++ stdenv.lib.optionals stdenv.isLinux [ libcap_ng systemd lmdb ]
+    ++ stdenv.lib.optional stdenv.isDarwin zlib; # perhaps due to gnutls
 
   enableParallelBuilding = true;