irqchip/gic-v3: Add missing include for barrier.h

Both the 32bit and 64bit versions of the GICv3 header file are using
barriers, but neglect to include barrier.h, leading to an interesting
splat in some circumstances.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1449483072-17694-3-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>