x86/x2apic: Acpi_gbl_FADT existence depends on CONFIG_ACPI

If ACPI is disabled, acpi_gbl_FADT is not available, and and the build
breaks.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Link: http://lkml.kernel.org/r/55479708.2000104@siemens.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic/x2apic_phys.c

@@ -21,11 +21,13 @@ early_param("x2apic_phys", set_x2apic_phys_mode);
 
 static bool x2apic_fadt_phys(void)
 {
+#ifdef CONFIG_ACPI
 	if ((acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID) &&
 		(acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL)) {
 		printk(KERN_DEBUG "System requires x2apic physical mode\n");
 		return true;
 	}
+#endif
 	return false;
 }