ACPI/processor_idle: Export acpi_processor_ffh_play_dead()

The kernel test robot reported the following build error:

  >> ERROR: modpost: "acpi_processor_ffh_play_dead" [drivers/acpi/processor.ko] undefined!

Caused by this recently merged commit:

  541ddf3 ("ACPI/processor_idle: Add FFH state handling")

The build failure is due to an oversight in the 'CONFIG_ACPI_PROCESSOR=m' case,
the function export is missing. Add it.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502151207.FA9UO1iX-lkp@intel.com/
Fixes: 541ddf3 ("ACPI/processor_idle: Add FFH state handling")
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/de5bf4f116779efde315782a15146fdc77a4a044.camel@linux.intel.com

arch/x86/kernel/acpi/cstate.c

@@ -214,6 +214,7 @@ void acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
 	percpu_entry = per_cpu_ptr(cpu_cstate_entry, cpu);
 	mwait_play_dead(percpu_entry->states[cx->index].eax);
 }
+EXPORT_SYMBOL_GPL(acpi_processor_ffh_play_dead);
 
 void __cpuidle acpi_processor_ffh_cstate_enter(struct acpi_processor_cx *cx)
 {