xen: follow changes to smp headers adding wbinvd

  commit c06b2760


  Author: Borislav Petkov <borislav.petkov@amd.com>
  Date:   Fri Jan 22 16:01:03 2010 +0100

    x86, lib: Add wbinvd smp helpers

Signed-off-by: Andy Whitcroft <apw@canonical.com>