Remove Michal Marek from Kbuild maintainers as there is no response from him
since October 2017.  Add an entry for Michal in CREDITS.

Michal, thanks for maintaining Kbuild for almost eight years!
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Signed-off-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>