ipv6: add ipv6_dev_find to stubs

Add ipv6_dev_find to ipv6_stub to allow lookup of net_devices by IPV6
address in net/ipv4/icmp.c.

Signed-off-by: Andreas Roeseler <andreas.a.roeseler@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>