netfilter: ebtables: allow xtables-nft only builds

Same patch as previous one, but for ebtables.

To build a kernel that only supports ebtables-nft, the builtin tables
need to be disabled, i.e.:

CONFIG_BRIDGE_EBT_BROUTE=n
CONFIG_BRIDGE_EBT_T_FILTER=n
CONFIG_BRIDGE_EBT_T_NAT=n

The ebtables specific extensions can then be used nftables'
NFT_COMPAT interface.

Signed-off-by: Florian Westphal <fw@strlen.de>