Motivation for this change

This already compiles fine, no changes necesssary.
I am currently working on getting DNF to compile on darwin (long story) and this is one of the less problematic dependencies :)

Example:

% /nix/store/svsyagf509zn9n00qc2nqja883vfbf5k-libmodulemd-2.9.2-bin/bin/modulemd-validator --help
Usage:
  modulemd-validator [OPTION…] FILES - Simple modulemd YAML validator

Help Options:
  -h, --help        Show help options

Application Options:
  --debug           Output debugging messages
  -q, --quiet       Print no output
  -v, --verbose     Be verbose
  -V, --version     Print version number, then exit

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.