include/kernel.mk: build kmod packages with empty FILES

include/kernel.mk: build kmod packages with empty FILES

kmod packages without FILES did not have an install step defined, leading
to no package being built. This affected netfilter/iptables packages, which
filter out builtin modules from FILES.

Not building a package that it is selected in .config is problematic, as
the generated empty package may be necessary to satisfy dependencies.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>