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>