netfilter: bridge: fix nf_tables bridge dependencies with main core

when CONFIG_NF_TABLES[_MODULE] is not enabled,
but CONFIG_NF_TABLES_BRIDGE is enabled:

net/bridge/netfilter/nf_tables_bridge.c: In function 'nf_tables_bridge_init_net':
net/bridge/netfilter/nf_tables_bridge.c:24:5: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c:25:9: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c:28:2: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c:30:34: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c:35:11: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c: In function 'nf_tables_bridge_exit_net':
net/bridge/netfilter/nf_tables_bridge.c:41:27: error: 'struct net' has no member named 'nft'
net/bridge/netfilter/nf_tables_bridge.c:42:11: error: 'struct net' has no member named 'nft'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>