Similarly to the previous patches by Sean Christopherson:

 "Through a labyrinthian sequence of includes, usage of virt_to_phys() is
  dependent on the include of asm/io.h in x86's asm/realmode.h, which is
  included in x86's asm/acpi.h and thus by linux/acpi.h.  Explicitly
  include linux/io.h to break the dependency on realmode.h so that a
  future patch can remove the realmode.h include from acpi.h without
  breaking the build."
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Link: https://lkml.kernel.org/r/157475520975.21853.16355518818746065226.tip-bot2@tip-bot2Signed-off-by: Ingo Molnar <mingo@kernel.org>