MAINTAINERS: Add linux-kbuild's patchwork

Add patchwork URL for Kconfig and Kbuild.

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