libcap: fix cross-compilation support

[Mindavi]

Description of changes

Between libcap 2.51 and 2.52, the CROSS_COMPILE flag appears in more places.
Maybe it was already required before, but now the build fails due to it not being set.

Set it unconditionally.

Things done

• Built on platform(s)
  • x86_64-linux (on staging-next for now, waiting for the staging build)
  • aarch64-linux (cross-compiled, on staging-next for now, waiting for the staging build)
  • x86_64-darwin
  • aarch64-darwin
• Check that out, dev and lib still contain the same files (with tree)
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.05 Release Notes (or backporting 21.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.