netfilter: nf_tables: flow_offload depends on flow_table

Without CONFIG_NF_FLOW_TABLE, the new nft_flow_offload module produces
a link error:

net/netfilter/nft_flow_offload.o: In function `nft_flow_offload_iterate_cleanup':
nft_flow_offload.c:(.text+0xb0): undefined reference to `nf_flow_table_iterate'
net/netfilter/nft_flow_offload.o: In function `flow_offload_iterate_cleanup':
nft_flow_offload.c:(.text+0x160): undefined reference to `flow_offload_dead'
net/netfilter/nft_flow_offload.o: In function `nft_flow_offload_eval':
nft_flow_offload.c:(.text+0xc4c): undefined reference to `flow_offload_alloc'
nft_flow_offload.c:(.text+0xc64): undefined reference to `flow_offload_add'
nft_flow_offload.c:(.text+0xc94): undefined reference to `flow_offload_free'

This adds a Kconfig dependency for it.

Fixes: a3c90f7a ("netfilter: nf_tables: flow offload expression")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>