netfilter: restore NF_INET_NUMHOOKS

This definition is used by the iptables legacy UAPI, restore it.

Fixes: d3519cb8 ("netfilter: nf_tables: add inet ingress support")
Reported-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>