x86: fix ACPI compile for LOCAL_APIC=n

ACPI processor idle code references local_apic_timer_c2_ok, which
is not available when LOCAL_APIC is disabled.

Define local_apic_timer_c2_ok as a constant, when LOCAL_APIC=n

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>