X86 ACPI: Use #ifdef not #if for CONFIG_X86 check

Fix a build warning on ia64:

include/linux/acpi.h:437:5: warning: "CONFIG_X86" is not defined
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/506f59ae9600b36a4@agluck-desktop.sc.intel.com
Cc: Len Brown <lenb@kernel.org>
Cc: Thomas Renninger <trenn@suse.de>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

include/linux/acpi.h

@@ -434,7 +434,7 @@ void acpi_os_set_prepare_sleep(int (*func)(u8 sleep_state,
 
 acpi_status acpi_os_prepare_sleep(u8 sleep_state,
 				  u32 pm1a_control, u32 pm1b_control);
-#if CONFIG_X86
+#ifdef CONFIG_X86
 void arch_reserve_mem_area(acpi_physical_address addr, size_t size);
 #else
 static inline void arch_reserve_mem_area(acpi_physical_address addr,