x86/reboot: Unconditionally define cpu_emergency_virt_cb typedef

Define cpu_emergency_virt_cb even if the kernel is being built without KVM
support so that KVM can reference the typedef in asm/kvm_host.h without
needing yet more #ifdefs.

No functional change intended.

Acked-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Tested-by: Farrah Chen <farrah.chen@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240830043600.127750-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>