x86/hyperv: Add HV_EXPOSE_INVARIANT_TSC define

Avoid open coding BIT(0) of HV_X64_MSR_TSC_INVARIANT_CONTROL by adding
a dedicated define. While there's only one user at this moment, the
upcoming KVM implementation of Hyper-V Invariant TSC feature will need
to use it as well.

Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221013095849.705943-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>