x86, irq: Fix build error caused by 9eabc99a

Commit 9eabc99a causes following build error when
IOAPIC is disabled.
   arch/x86/pci/irq.c: In function 'pirq_disable_irq':
>> arch/x86/pci/irq.c:1259:2: error: implicit declaration of function 'mp_should_keep_irq' [-Werror=implicit-function-declaration]
     if (io_apic_assign_pci_irqs && !mp_should_keep_irq(&dev->dev) &&
     ^
   cc1: some warnings being treated as errors

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Grant Likely <grant.likely@linaro.org>
Link: http://lkml.kernel.org/r/1409382916-10649-1-git-send-email-jiang.liu@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>