LinuxKPI: add cleanup.h for guard DEFINE_GUARD/guard.

iwlwifi v6.11 started to use guard(). _T in the implementation needs
to be exposed to the driver which uses it in DEFINE_GUARD().

Given this is the first instance of attribute(cleanup) in the tree:

• gcc has support for it since at least 2003-06-04 (0bfa5f65bfb186f10d43304946fd7fcd69988732)
• llvm gained support on 2009-01-31 (d277d790e0f6f23043397ba919619b5c3e157ff3, llvm-svn 63462).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D46843