x86: irq: Fix placement of mp_should_keep_irq()

While f3761db1 ("x86, irq: Fix build error caused by
9eabc99a

") addressed the original build problem,
declaration, inline stub, and definition still seem misplaced: It isn't
really IO-APIC related, and it's being used solely in arch/x86/pci/.
This also means stubbing it out when !CONFIG_X86_IO_APIC was at least
questionable.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Link: http://lkml.kernel.org/r/545747BE020000780004436E@mail.emea.novell.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>