ACPI: fix section mismatch in processor_core.c:acpi_processor_hotplug_notify

Fix following warning:
WARNING: vmlinux.o(.text+0x55586c): Section mismatch in reference from the function acpi_processor_hotplug_notify() to the function .cpuinit.text:acpi_processor_start()

acpi_processor_hotplug_notify() may safely reference __cpuinit
stuff as it ids defined inside an ACPI_HOTPLUG_CPU block.
So annotate it __ref to silence the warning.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Len Brown <len.brown@intel.com>