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 --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4be570a..7de3d67 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -566,7 +566,7 @@
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
 	nf_conntrack_put(skb->nfct);
 #endif
-#ifdef CONFIG_BRIDGE_NETFILTER
+#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 	nf_bridge_put(skb->nf_bridge);
 #endif
 /* XXX: IS this still necessary? - JHS */