xen: resolve section mismatch warnings in xen-acpi-processor

The following resolves a section mismatch warning below in xen-acpi-processor introduced by
3fac1014 [13/13] xen: Re-upload processor PM data to hypervisor after S3 resume (v2)

Warning:
WARNING: drivers/xen/built-in.o(.text+0x2056a): Section mismatch in reference from the function xen_upload_processor_pm_data() to the function .init.text:read_acpi_id()
   The function xen_upload_processor_pm_data() references
   the function __init read_acpi_id().
   This is often because xen_upload_processor_pm_data lacks a __init
   annotation or the annotation of read_acpi_id is wrong.

Reported-by: kbuild test robot <fengguang.wu@intel.com>

Signed-off-by: Ben Guthro <benjamin.guthro@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>