readline: add library symlinks on OpenBSD

[rhelmot]

This install error is caused by a very old libtool. We can't autoreconfHook this package, so this is the best we've got!

Before:

$ ll /nix/store/q4xfivjrnjhshp0khjy66nq5ljjdmfv1-readline-x86_64-unknown-openbsd-8.2p13/lib
total 448K
-r--r--r-- 1 root root  48K Dec 31  1969 libhistory.so.8.2
-r--r--r-- 1 root root 399K Dec 31  1969 libreadline.so.8.2

After:

$ ll /nix/store/q9986qvm5jhnlxckp972cmhdgp5v52km-readline-x86_64-unknown-openbsd-8.2p13/lib
total 448K
lrwxrwxrwx 1 root root   17 Dec 31  1969 libhistory.so -> libhistory.so.8.2
-r--r--r-- 1 root root  48K Dec 31  1969 libhistory.so.8.2
lrwxrwxrwx 1 root root   18 Dec 31  1969 libreadline.so -> libreadline.so.8.2
-r--r--r-- 1 root root 399K Dec 31  1969 libreadline.so.8.2

Things done

• Built on platform(s)
  • [x] x86_64-linux (openbsd cross)
  • [ ] 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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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
• [x] Fits CONTRIBUTING.md.

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