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>

drivers/ptp/ptp_kvm.c

@@ -22,7 +22,7 @@ struct kvm_ptp_clock {
 	struct ptp_clock_info caps;
 };
 
-DEFINE_SPINLOCK(kvm_ptp_lock);
+static DEFINE_SPINLOCK(kvm_ptp_lock);
 
 static struct pvclock_vsyscall_time_info *hv_clock;