modpost: disallow the combination of EXPORT_SYMBOL and __meminit*

Theoretically, we could export conditionally-discarded code sections,
such as .meminit*, if all the users can become modular under a certain
condition. However, that would be difficult to control and such a tricky
case has never occurred.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>