libclc: fix libclc.pc file

[kotatsuyaki]

Closes #152060

Apply the same fix as in #81091. See also the tracking issue #144170.

Motivation for this change

The wrong include path in the pkgconfig script caused OpenCL kernel compile failing with mesa (see #152060).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• 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.

[r-burns]

I opened an upstream revision for this, which may be a cleaner fix: https://reviews.llvm.org/D118959

This looks like a reasonable workaround to me. Please target the staging branch due to the high rebuild count.

[r-burns]

LGTM