irqchip/gic-v3: Add missing struct device_node declaration

When the GICv3 header file is used in a C file that doesn't include
any of the OF stuff, we end up with a bunch of ugly warnings.

Let's keep GCC quiet by adding a forward declaration.
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/1449483072-17694-2-git-send-email-marc.zyngier@arm.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irqchip/arm-gic-v3.h

@@ -330,6 +330,7 @@ struct rdists {
 };
 
 struct irq_domain;
+struct device_node;
 int its_cpu_init(void);
 int its_init(struct device_node *node, struct rdists *rdists,
 	     struct irq_domain *domain);