sh: mach-x3proto: needs linux/irq.h.

Since commit ba84be23
("remove linux/hardirq.h from asm-generic/local.h"), the asm/irq.h
definitions that mach-x3proto/setup.c depends on are no longer available,
causing the build to die. So, include linux/irq.h explicitly.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>