libprocstat: Add static assertions for kinfo structure sizes

[jhb]

Summary

This ensures the structure sizes are validated for architectures that
do not build a kernel.

[kib]

I like this, but I have a small request. I am curious if you agree with it.

Can we have a separate source file in libprocstat which only consists of assertions? This is from my experience with maintaining the asserts in kern_thread.c, where to get past compilation I have to fix the layout constants first, which was a secondary to get the code compiled.
It is less important for libprocstat since kinfo must not change often, but IMO it is worth it by being able to temporary remove the assert file from the compilation.

Anyway I am fine with the change as is as well.