cargo-expand: fix darwin build

pkgs/development/tools/rust/cargo-expand/default.nix

@@ -1,4 +1,4 @@
-{ lib, rustPlatform, fetchFromGitHub }:
+{ lib, rustPlatform, fetchFromGitHub, stdenv, libiconv }:
 
 rustPlatform.buildRustPackage rec {
   pname = "cargo-expand";
@@ -13,6 +13,8 @@ rustPlatform.buildRustPackage rec {
 
   cargoSha256 = "sha256-1+A+n5VQS8zJULiR8IWLGo+RnFuVjg6ist8G3eCsXJM=";
 
+  buildInputs = lib.optional stdenv.isDarwin libiconv;
+
   meta = with lib; {
     description =
       "A utility and Cargo subcommand designed to let people expand macros in their Rust source code";