darwin: gnutls: add missing libiconv dep

pkgs/development/libraries/gnutls/generic.nix

@@ -1,5 +1,5 @@
 { lib, fetchurl, stdenv, zlib, lzo, libtasn1, nettle, pkgconfig, lzip
-, guileBindings, guile, perl, gmp, autogen, libidn, p11_kit, unbound
+, guileBindings, guile, perl, gmp, autogen, libidn, p11_kit, unbound, libiconv
 , tpmSupport ? false, trousers
 
 # Version dependent args
@@ -30,6 +30,7 @@ stdenv.mkDerivation {
   enableParallelBuilding = !guileBindings;
 
   buildInputs = [ lzo lzip nettle libtasn1 libidn p11_kit zlib gmp autogen ]
+    ++ lib.optional (stdenv.isDarwin) libiconv
     ++ lib.optional (tpmSupport && stdenv.isLinux) trousers
     ++ [ unbound ]
     ++ lib.optional guileBindings guile;