linuxkpi: Annotate an unused variable as such

This addresses a -Wunused-but-set-variable warning in the gcc builds.

No functional change intended.

Reported by: Jenkins

LinuxKPI: Add static_assert() macro

And move it along BUILD_BUG family in to
linux/build_bug.h to match Linux.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42800