netfilter: nf_tables: Support auto-loading for inet nat

Trying to create an inet family nat chain would not cause
nft_chain_nat.ko module to auto-load due to missing module alias. Add a
proper one with hard-coded family value 1 for the pseudo-family
NFPROTO_INET.

Fixes: d164385e ("netfilter: nat: add inet family nat support")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>