netfilter: nf_tables: drop include of module.h from nf_tables.h

Ideally, header files under include/linux shouldn't be adding
includes of other headers, in anticipation of their consumers,
but just the headers needed for the header itself to pass
parsing with CPP.

The module.h is particularly bad in this sense, as it itself does
include a whole bunch of other headers, due to the complexity of
module support.

Since nf_tables.h is not going into a module struct looking for
specific fields, we can just let it know that module is a struct,
just like about 60 other include/linux headers already do.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>