KVM: selftests: change pthread_yield to sched_yield

With later GCC we get

 steal_time.c: In function ‘main’:
 steal_time.c:323:25: warning: ‘pthread_yield’ is deprecated: pthread_yield is deprecated, use sched_yield instead [-Wdeprecated-declarations]

Let's follow the instructions and use sched_yield instead.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210713203742.29680-2-drjones@redhat.com