Kbuild: Makefile.modbuiltin: include auto.conf and tristate.conf mandatory

The files auto.conf and tristate.conf are mandatory for building
modules.builtin files, therefore include them as such.

Usually, the top-level Makefile ensures that those files exist but we
want to make sure we get noticed if they are missing for whatever
reason.

Signed-off-by: Dirk Gouders <dirk@gouders.net>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>