tipc: fix compile error when IPV6=m and TIPC=y

When IPV6=m and TIPC=y, below error will appear during building kernel
image:

net/tipc/udp_media.c:196:
undefined reference to `ip6_dst_lookup'
make: *** [vmlinux] Error 1

As ip6_dst_lookup() is implemented in IPV6 and IPV6 is compiled as
module, ip6_dst_lookup() is not built-in core kernel image. As a
result, compiler cannot find 'ip6_dst_lookup' reference while
compiling TIPC code into core kernel image.

But with the method introduced by commit 5f81bd2e ("ipv6: export a
stub for IPv6 symbols used by vxlan"), we can avoid the compile error
through "ipv6_stub" pointer to access ip6_dst_lookup().

Fixes: d0f91938 ("tipc: add ip/udp media type")
Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>