Fix usb-cam build

[IamTheCarl]

This package was failing to build because it was missing pkg-config.
I have not tested it on any distros other than humble, but I can add it and see if they at least build.

[wentasah]

Thanks for the fix, but please don't modify the automatically generated files. The best fix is to add pkg-config to the upsteam package.xml. If that's not possible or not fast enough, add an override to overrides.nix. If multiple distros need the fix, it's IMHO the best to add the override to distro-overlay.nix as in #511 (comment).

[IamTheCarl]

You're welcome, and thank you for taking the time to review it. If I had read the README more closely, I would have realized this was the wrong way to go about things. A header at the top of generated files stating "Do not modify, this file was auto generated by superflore" would have clued me in a little harder.

I'm not closing out this PR yet because I don't know if upstream will accept, reject, or ignore my PR. If the PR is not accepted, I will rework this PR to use the distro-overlay.nix alternative you suggested.