mkpasswd: fix build with clang

[elohmeier]

Description of changes

Fixes the mkpasswd build on darwin based on #260791.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[giomf]

Result of nixpkgs-review pr 265824 run on x86_64-linux 1

4 packages built:

• automysqlbackup
• mailutils
• mailutils.debug
• mkpasswd

[github-actions]

Backport failed for release-23.11, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-23.11
git worktree add -d .worktree/backport-265824-to-release-23.11 origin/release-23.11
cd .worktree/backport-265824-to-release-23.11
git switch --create backport-265824-to-release-23.11
git cherry-pick -x 307ef68a01a6c2cc6ab932ef765a2909743130a3