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

tools/testing/selftests/kvm/steal_time.c

@@ -320,7 +320,7 @@ int main(int ac, char **av)
 		run_delay = get_run_delay();
 		pthread_create(&thread, &attr, do_steal_time, NULL);
 		do
-			pthread_yield();
+			sched_yield();
 		while (get_run_delay() - run_delay < MIN_RUN_DELAY_NS);
 		pthread_join(thread, NULL);
 		run_delay = get_run_delay() - run_delay;