net: fix some compiler warning in net/core/neighbour.c

net/core/neighbour.c:65:12: warning: 'zero' defined but not used [-Wunused-variable]
net/core/neighbour.c:66:12: warning: 'unres_qlen_max' defined but not used [-Wunused-variable]

These variables are only used when CONFIG_SYSCTL is defined,
so move them under #ifdef CONFIG_SYSCTL.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Acked-by: Shan Wei <davidshan@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>