[IPV6]: Make the ipv6/sysctl_net_ipv6.c compilation cleaner

Since this file is entirely enclosed with the
#ifdef CONFIG_SYSCTL/#endif pair, it's OK to move this
CONFIG_ into a Makefile.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>