tcp: Fix build error if IPV6 is not selected

If CONFIG_IPV6=m is selected, we are getting following build errors.

net/built-in.o: In function `tcp_is_local6':
net/ipv4/tcp.c:3261: undefined reference to `rt6_lookup'

Making the code conditional upon only CONFIG_IPV6=y fixes this issue.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
CC: John Stultz <john.stultz@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 880d2a6..7921d1a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3471,7 +3471,7 @@
 	return rt->dst.dev && (rt->dst.dev->flags & IFF_LOOPBACK);
 }
 
-#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
+#if defined(CONFIG_IPV6)
 static int tcp_is_local6(struct net *net, struct in6_addr *addr) {
 	struct rt6_info *rt6 = rt6_lookup(net, addr, addr, 0, 0);
 	return rt6 && rt6->dst.dev && (rt6->dst.dev->flags & IFF_LOOPBACK);
@@ -3528,7 +3528,7 @@
 					continue;
 			}
 
-#if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE)
+#if defined(CONFIG_IPV6)
 			if (family == AF_INET6) {
 				struct in6_addr *s6;
 				if (!inet->pinet6)