netfilter: nf_tables: incorrect enum nft_list_attributes definition

This should be NFTA_LIST_UNSPEC instead of NFTA_LIST_UNPEC, all other
similar attribute definitions are postfixed with _UNSPEC.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>