netfilter: bridge: build br_nf_core only if required

Eric reports build failure with
CONFIG_BRIDGE_NETFILTER=n

We insist to build br_nf_core.o unconditionally, but we must only do so
if br_netfilter was enabled, else it fails to build due to
functions being defined to empty stubs (and some structure members
being defined out).

Also, BRIDGE_NETFILTER=y|m makes no sense when BRIDGE=n.

Fixes: 34666d46 (netfilter: bridge: move br_netfilter out of the core)
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>