[IPV6]: Fix sysctl compilation error.

Move ipv6_icmp_sysctl_init and ipv6_route_sysctl_init into the right
ifdef section otherwise that does not compile when CONFIG_SYSCTL=yes
and CONFIG_PROC_FS=no

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>