genirq: Include missing header file in irqdomain.c

Include appropriate header file include/linux/of_irq.h in
kernel/irq/irqdomain.c because it contains prototype definition of
function define in kernel/irq/irqdomain.c.

This eliminates the following warning in kernel/irq/irqdomain.c:
kernel/irq/irqdomain.c:468:14: warning: no previous prototype for ‘irq_create_of_mapping’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Link: http://lkml.kernel.org/r/eb89aebea7ff1a46122918ac389ebecf8248be9a.1393493276.git.rashika.kheria@gmail.com


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