ipv6: Move <linux/in6.h> from header-y to unifdef-y.

Given that <linux/in6.h> contains a __KERNEL__ test, it should be
unifdef-ed.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>