x86: io_apic: Avoid unused variable warning when CONFIG_GENERIC_PENDING_IRQ=n

arch/x86/kernel/apic/io_apic.c: In function 'ack_apic_level':
arch/x86/kernel/apic/io_apic.c:2433: warning: unused variable 'desc'

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <201010272107.o9RL7rse018212@imap1.linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>