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>