[IPV6]: Kill export of fl6_sock_lookup.

There is no usage of this EXPORT_SYMBOL in the kernel.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>