guile: add libiconv as buildInput on darwin

[LnL7]

Motivation for this change

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • OS X
  • Np changes on other platforms
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[mention-bot]

@LnL7, thanks for your PR! By analyzing the history of the files in this pull request, we identified @civodul, @edolstra and @lovek323 to be potential reviewers.

[copumpkin]

Can you elaborate on why guile works for me without this on Darwin?

Compare to #19470 (comment)

[LnL7]

I was just going to ask you, I've seen a bunch of failures on hydra with the same issue.

[copumpkin]

Thank you both for tackling this so quickly!

[civodul]

Mention Bot notifications@github.com skribis:

@LnL7, thanks for your PR! By analyzing the history of the files in this pull request, we identified @civodul, @edolstra and @lovek323 to be potential reviewers.

It seems Mention Bot has ignored to ignore me again. Could you look
into it?

TIA! :-)

Ludo’.

[domenkozar]

@civodul I've opened facebookarchive/mention-bot#178 to address this. Sorry for inconvenience :)