shaderc: fix pc file generation

[marcin-serwin]

I should have written the proper patch the first time, sorry for that.

Fixes #477464 (comment)

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[K900]

Can we send this to staging-not-next? AFAICT ffmpeg is the only failure on next, and it's a lot of rebuilds.

[marcin-serwin]

The broken pc files should be generally harmless, it seems like ffmpeg just does its own postprocessing on the pkg-config output which causes the incorrect interpretation. I think this can wait on staging.