ndisc: Remove unused ndisc_ifinfo_sysctl_strategy() declaration

Commit f8572d8f ("sysctl net: Remove unused binary sysctl code")
left behind this declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>