x86, irq, ACPI: Fix building warning of unused code

Fix building warning when CONFIG_X86_LOCAL_APIC is disabled.
arch/x86/kernel/acpi/boot.c:699:20: warning: ‘acpi_set_irq_model_ioapic’ defined but not used [-Wunused-function]

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Link: http://lkml.kernel.org/r/1414397531-28254-3-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/acpi/boot.c

@@ -690,6 +690,7 @@ void acpi_unregister_gsi(u32 gsi)
 }
 EXPORT_SYMBOL_GPL(acpi_unregister_gsi);
 
+#ifdef CONFIG_X86_LOCAL_APIC
 static void __init acpi_set_irq_model_ioapic(void)
 {
 	acpi_irq_model = ACPI_IRQ_MODEL_IOAPIC;
@@ -697,6 +698,7 @@ static void __init acpi_set_irq_model_ioapic(void)
 	__acpi_unregister_gsi = acpi_unregister_gsi_ioapic;
 	acpi_ioapic = 1;
 }
+#endif
 
 /*
  *  ACPI based hotplug support for CPU