boringssl: add the "dev" output to match openssl

[bobrik]

Motivation for this change

This allows building curl with a openssl = boringssl override. Tested by building curl with the boringssl override and nginxQuic.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages 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/)
• 21.11 Release Notes (or backporting 21.05 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.

[SuperSandro2000]

If all packages would use lib.getDev instead of openssl.dev this would just work ™️ .

[bobrik]

Sounds reasonable: #146646.

[roberth]

From the looks of it, this was a good change, reducing runtime closure size a little.

[bobrik]

I can reopen if you want to merge it.

[roberth]

Thanks!