MAINTAINERS: Add Nathan and Nicolas to Kbuild reviewers

As suggested by Nick, add Nathan and myself to Kbuild reviewers to share more
review responsibilities.

Signed-off-by: Nicolas Schier <nicolas@fjasle.eu>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>