[NETFILTER]: Fix build by putting back ip_nat_lock extern in ip_nat.h

Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter_ipv4/ip_nat.h

@@ -52,6 +52,9 @@ struct ip_nat_multi_range_compat
 #include <linux/list.h>
 #include <linux/netfilter_ipv4/lockhelp.h>
 
+/* Protects NAT hash tables, and NAT-private part of conntracks. */
+DECLARE_RWLOCK_EXTERN(ip_nat_lock);
+
 /* The structure embedded in the conntrack structure. */
 struct ip_nat_info
 {