MAINTAINERS: new kbuild maintainer

Sam was fine with handing over kbuild maintainership to me. The git
trees are already in linux-next, a merge request will follow shortly.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>