x86/apic: Only compile local function if used with !CONFIG_GENERIC_PENDING_IRQ

x86/apic: Only compile local function if used with !CONFIG_GENERIC_PENDING_IRQ

The local function io_apic_level_ack_pending() is only called
from io_apic_level_ack_pending(). The later function is only
compiled if CONFIG_GENERIC_PENDING_IRQ is defined. Move the
io_apic_level_ack_pending() to the existing #ifdef
CONFIG_GENERIC_PENDING_IRQ code block.

This will remove the following warning message during compiling
without CONFIG_GENERIC_PENDING_IRQ defined:

 * arch/x86/kernel/apic/io_apic.c:382: warning: ‘io_apic_level_ack_pending’ defined but not used

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Naga Chumbalkar <nagananda.chumbalkar@hp.com>
Link: http://lkml.kernel.org/r/1336461860.2296.3.camel@sbsiddha-mobl2
Signed-off-by: Ingo Molnar <mingo@kernel.org>