cmake, ci: Lint with include-what-you-use

[langston-barrett]

https://include-what-you-use.org/

[langston-barrett]

I forgot that this tool doesn't always generate high-quality suggestions...

CAVEAT This is alpha quality software -- at best (as of July 2018).

https://github.com/include-what-you-use/include-what-you-use/blob/master/README.md#caveat

Looks like pragmas might help, though:

https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md