gping: fix darwin build

pkgs/tools/networking/gping/default.nix

@@ -1,6 +1,8 @@
 { lib
+, stdenv
 , rustPlatform
 , fetchFromGitHub
+, libiconv
 }:
 
 rustPlatform.buildRustPackage rec {
@@ -16,6 +18,8 @@ rustPlatform.buildRustPackage rec {
 
   cargoSha256 = "sha256-2PxhtAqROgufVGGH7VtEJJU6Sa2OrGbbMVRUWYbAD0Q=";
 
+  buildInputs = lib.optional stdenv.isDarwin libiconv;
+
   meta = with lib; {
     description = "Ping, but with a graph";
     homepage = "https://github.com/orf/gping";