netfilter: nf_tables: add missing prototypes.

Sparse rightly complains about undeclared symbols.

  CHECK   net/netfilter/nft_set_hash.c
net/netfilter/nft_set_hash.c:647:21: warning: symbol 'nft_set_rhash_type' was not declared. Should it be static?
net/netfilter/nft_set_hash.c:670:21: warning: symbol 'nft_set_hash_type' was not declared. Should it be static?
net/netfilter/nft_set_hash.c:690:21: warning: symbol 'nft_set_hash_fast_type' was not declared. Should it be static?
  CHECK   net/netfilter/nft_set_bitmap.c
net/netfilter/nft_set_bitmap.c:296:21: warning: symbol 'nft_set_bitmap_type' was not declared. Should it be static?
  CHECK   net/netfilter/nft_set_rbtree.c
net/netfilter/nft_set_rbtree.c:470:21: warning: symbol 'nft_set_rbtree_type' was not declared. Should it be static?

Include nf_tables_core.h rather than nf_tables.h to pick up the additional definitions.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>