xfrm4: fix build when SYSCTLs are disabled

Fix build errors when SYSCTLs are not enabled:
(.init.text+0x5154): undefined reference to `net_ipv4_ctl_path'
(.init.text+0x5176): undefined reference to `register_net_sysctl_table'
xfrm4_policy.c:(.exit.text+0x573): undefined reference to `unregister_net_sysctl_table

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>