KVM: selftests: Make hyperv_clock require TSC based system clocksource

KVM sets up Hyper-V TSC page clocksource for its guests when system
clocksource is 'based on TSC' (see gtod_is_based_on_tsc()), running
hyperv_clock with any other clocksource leads to imminent failure.

Add the missing requirement to make the test skip gracefully.

Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/x86_64/hyperv_clock.c

···
 	int stage;
 
 	TEST_REQUIRE(kvm_has_cap(KVM_CAP_HYPERV_TIME));
+	TEST_REQUIRE(sys_clocksource_is_based_on_tsc());
 
 	vm = vm_create_with_one_vcpu(&vcpu, guest_main);