modpost: show warning if vmlinux is not found when processing modules

check_exports() does not print warnings about unresolved symbols if
vmlinux is missing because there would be too many.

This situation happens when you do 'make modules' from the clean
tree, or compile external modules against a kernel tree that has
not been completely built.

It is dangerous to not check unresolved symbols because you might be
building useless modules. At least it should be warned.

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