svd2rust: fix darwin build

pkgs/development/tools/rust/svd2rust/default.nix

@@ -1,4 +1,4 @@
-{ lib, fetchFromGitHub, rustPlatform }:
+{ lib, stdenv, fetchFromGitHub, rustPlatform, libiconv }:
 
 with rustPlatform;
 
@@ -16,6 +16,8 @@ buildRustPackage rec {
 
   cargoSha256 = "0c0f86x17fzav5q76z3ha3g00rbgyz2lm5a5v28ggy0jmg9xgsv6";
 
+  buildInputs = lib.optional stdenv.isDarwin libiconv;
+
   meta = with lib; {
     description = "Generate Rust register maps (`struct`s) from SVD files";
     homepage = "https://github.com/rust-embedded/svd2rust";