x86: Fix out of bounds irq handlers access

Using coreboot-x86_defconfig, the following error occurred prior to this modification:
CC	arch/x86/lib/interrupts
arch/x86/lib/interrupts.c: In function ‘do_irqinfo’:
arch/x86/lib/interrupts.c:134:24: error: iteration 16u invokes undefined behavior [-Werror=aggressive-loop-optimizations]
   if (irq_handlers[irq].handler != NULL) {
                        ^
arch/x86/lib/interrupts.c:133:2: note: containing loop
  for (irq = 0; irq <= CONFIG_SYS_NUM_IRQS; irq++) {
  ^
cc1: all warnings being treated as errors
scripts/Makefile.build:275: recipe for target 'arch/x86/lib/interrupts.o' failed
make[1]: *** [arch/x86/lib/interrupts.o] Error 1
Makefile:1093: recipe for target 'arch/x86/lib' failed
make: *** [arch/x86/lib] Error 2

Change-Id: I3572a822081b72ab760f1eb99442e1161d3d167e
Signed-off-by: Sebastien Ronsse <sronsse@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>