netfilter: use IS_ENABLED(CONFIG_BRIDGE_NETFILTER)

In 34666d4 ("netfilter: bridge: move br_netfilter out of the core"),
the bridge netfilter code has been modularized.

Use IS_ENABLED instead of ifdef to cover the module case.

Fixes: 34666d4 ("netfilter: bridge: move br_netfilter out of the core")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --cc net/core/skbuff.c
Simple merge

diff --cc net/ipv4/ip_output.c
Simple merge

diff --cc net/ipv4/netfilter/ipt_REJECT.c
Simple merge

diff --cc net/ipv4/netfilter/nf_defrag_ipv4.c
Simple merge

diff --cc net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
Simple merge

diff --cc net/netfilter/ipset/ip_set_hash_netiface.c
Simple merge

diff --cc net/netfilter/nf_log_common.c
Simple merge

diff --cc net/netfilter/nf_queue.c
Simple merge

diff --cc net/netfilter/nfnetlink_log.c
Simple merge

diff --cc net/netfilter/nfnetlink_queue_core.c
Simple merge