x86/hyperv: Hide pv_ops access for CONFIG_PARAVIRT=n

hv_setup_sched_clock() references pv_ops which is only available when
CONFIG_PARAVIRT=Y.

Wrap it into a #ifdef

Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190828080747.204419-1-Tianyu.Lan@microsoft.com