x86: fix !SMP compiler warning in arch/x86/kernel/acpi/processor.c

Fix !CONFIG_SMP warning:

arch/x86/kernel/acpi/processor.c: In function arch_acpi_processor_init_pdc:
arch/x86/kernel/acpi/processor.c:65: warning: unused variable cpu

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>