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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/Kbuild

···
 header-y += if_strip.h
 header-y += if_tun.h
 header-y += if_tunnel.h
-header-y += in6.h
 header-y += in_route.h
 header-y += ioctl.h
 header-y += ip6_tunnel.h
···
 unifdef-y += igmp.h
 unifdef-y += inet_diag.h
 unifdef-y += in.h
+unifdef-y += in6.h
 unifdef-y += inotify.h
 unifdef-y += input.h
 unifdef-y += ip.h