Add libiconv to regress module buildInputs.

pkgs/development/python-modules/regress/default.nix

@@ -1,7 +1,9 @@
 { lib
+, stdenv
 , fetchPypi
 , buildPythonPackage
 , rustPlatform
+, libiconv
 }:
 
 buildPythonPackage rec {
@@ -17,6 +19,8 @@ buildPythonPackage rec {
 
   nativeBuildInputs = with rustPlatform; [ cargoSetupHook maturinBuildHook ];
 
+  buildInputs = lib.optionals stdenv.hostPlatform.isDarwin [ libiconv ];
+
   cargoDeps = rustPlatform.fetchCargoTarball {
     inherit src;
     name = "${pname}-${version}";