ACPI: processor_idle: Silence missing prototype warnings

Silence the following warnings when built with W=1:

 | CC   drivers/acpi/processor_idle.c
 |      warning: no previous prototype for 'acpi_processor_ffh_lpi_probe' [-Wmissing-prototypes]
 |		int __weak acpi_processor_ffh_lpi_probe(unsigned int cpu)
 |                         ^
 | CC   drivers/acpi/processor_idle.c
 |      warning: no previous prototype for 'acpi_processor_ffh_lpi_enter' [-Wmissing-prototypes]
 |              int __weak acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi)
 |                         ^
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/acpi/processor.h

@@ -460,4 +460,9 @@ static inline void acpi_thermal_cpufreq_exit(struct cpufreq_policy *policy)
 }
 #endif	/* CONFIG_CPU_FREQ */
 
+#ifdef CONFIG_ACPI_PROCESSOR_IDLE
+extern int acpi_processor_ffh_lpi_probe(unsigned int cpu);
+extern int acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi);
+#endif
+
 #endif