kbuild: add net/netfilter/nf_tables_offload.h to header-test blacklist.

net/netfilter/nf_tables_offload.h includes net/netfilter/nf_tables.h
which is itself on the blacklist.

Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>