x86: probe nr_irqs even only mptable is used

for !CONFIG_HAVE_SPARSE_IRQ

fix:

 In file included from arch/x86/kernel/early-quirks.c:18:
 include/asm/io_apic.h: In function 'probe_nr_irqs':
 include/asm/io_apic.h:209: error: 'NR_IRQS' undeclared (first use in this function)
 include/asm/io_apic.h:209: error: (Each undeclared identifier is reported only once
 include/asm/io_apic.h:209: error: for each function it appears in.)

v2: fix by Ingo

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>