KVM: selftests: Add missing header file needed by xAPIC IPI tests

Fixes: 678e90a3

 ("KVM: selftests: Test IPI to halted vCPU in xAPIC while backing page moves")
Cc: Andrew Jones <drjones@redhat.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Peter Shier <pshier@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210210011747.240913-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>