kbuild: force to build vmlinux if CONFIG_MODVERSION=y

This code does not work as stated in the comment.

$(CONFIG_MODVERSIONS) is always empty because it is expanded before
include/config/auto.conf is included. Hence, 'make modules' with
CONFIG_MODVERSION=y cannot record the version CRCs.

This has been broken since 2003, commit ("kbuild: Enable modules to be
build using the "make dir/" syntax"). [1]

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=15c6240cdc44bbeef3c4797ec860f9765ef4f1a7


Cc: linux-stable <stable@vger.kernel.org> # v2.5.71+
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>