[NETFILTER]: Fix compile with NAT but without modules

Here's another patch to fix compile-errors, this time with NAT but
without modules. There's a missing 'extern' in ip_conntrack_tftp.h
Signed-off-by: Martin Josefsson <gandalf@wlug.westbo.se>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter_ipv4/ip_conntrack_tftp.h

@@ -13,7 +13,7 @@ struct tftphdr {
 #define TFTP_OPCODE_ACK		4
 #define TFTP_OPCODE_ERROR	5
 
-unsigned int (*ip_nat_tftp_hook)(struct sk_buff **pskb,
+extern unsigned int (*ip_nat_tftp_hook)(struct sk_buff **pskb,
 				 enum ip_conntrack_info ctinfo,
 				 struct ip_conntrack_expect *exp);