netfilter: bridge: make NF_TABLES_BRIDGE tristate

The new nft_meta_bridge code fails to link as built-in when NF_TABLES
is a loadable module.

net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_get_eval':
nft_meta_bridge.c:(.text+0x1e8): undefined reference to `nft_meta_get_eval'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_get_init':
nft_meta_bridge.c:(.text+0x468): undefined reference to `nft_meta_get_init'
nft_meta_bridge.c:(.text+0x49c): undefined reference to `nft_parse_register'
nft_meta_bridge.c:(.text+0x4cc): undefined reference to `nft_validate_register_store'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_module_exit':
nft_meta_bridge.c:(.exit.text+0x14): undefined reference to `nft_unregister_expr'
net/bridge/netfilter/nft_meta_bridge.o: In function `nft_meta_bridge_module_init':
nft_meta_bridge.c:(.init.text+0x14): undefined reference to `nft_register_expr'
net/bridge/netfilter/nft_meta_bridge.o:(.rodata+0x60): undefined reference to `nft_meta_get_dump'
net/bridge/netfilter/nft_meta_bridge.o:(.rodata+0x88): undefined reference to `nft_meta_set_eval'

This can happen because the NF_TABLES_BRIDGE dependency itself is just a
'bool'.  Make the symbol a 'tristate' instead so Kconfig can propagate the
dependencies correctly.

Fixes: 30e103fe

 ("netfilter: nft_meta: move bridge meta keys into nft_meta_bridge")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>