They don't ship the header files, you need to supply them yourself by building grok (and likely install them in the system-wide headers path).
Just an fyi, grok hasn't been touched in 7-10 years, go-grok since 2014. I'm not really sure what you're trying to do, just in general be mindful that you're going to be using a dep which is no longer in development (and likely is never going to see future development)
Also this has nothing to do with Support or Installation of Solus.