cpp: add missing includes (fixes gcc build)

[Bvngee]

This fixes building material-color-utilities with gcc.

I know it says this repo is not currently accepting contributions; feel free to close this and add the three lines yourself. It was just easier than an issue (I can do that too if desired though).

cc maintainers (I hope these are right): @cushon @kluever

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[kluever]

I may have made a small commit at some point, but I'm not a maintainer of this project.

/cc @material-admin @guidezpl