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 <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/Makefile.modbuiltin

···
 PHONY := __modbuiltin
 __modbuiltin:
 
--include include/config/auto.conf
+include include/config/auto.conf
 # tristate.conf sets tristate variables to uppercase 'Y' or 'M'
 # That way, we get the list of built-in modules in obj-Y
--include include/config/tristate.conf
+include include/config/tristate.conf
 
 include scripts/Kbuild.include