kbuild: deb-pkg: do not build linux-headers package if CONFIG_MODULES=n

kbuild: deb-pkg: do not build linux-headers package if CONFIG_MODULES=n

Since commit 269a535ca931 ("modpost: generate vmlinux.symvers and
reuse it for the second modpost"), with CONFIG_MODULES disabled,
"make deb-pkg" (or "make bindeb-pkg") fails with:

  find: ‘Module.symvers’: No such file or directory

If CONFIG_MODULES is disabled, it doesn't really make sense to build
the linux-headers package.

Fixes: 269a535ca931 ("modpost: generate vmlinux.symvers and reuse it for the second modpost")
Reported-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>