ptp_kvm: Make kvm_ptp_lock static

Fix sparse warning:

drivers/ptp/ptp_kvm.c:25:1: warning:
 symbol 'kvm_ptp_lock' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>