irqchip/gic-v2m: Fix missing include of <linux/irqchip/arm-gic.h>

irqchip/gic-v2m: Fix missing include of <linux/irqchip/arm-gic.h>

Fix the missing declaration of gicv2m_init() by including the
file <linux/irqchip/arm-gic.h> which defines it. Fixes the
warning:

drivers/irqchip/irq-gic-v2m.c:517:12: warning: symbol 'gicv2m_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lkml.kernel.org/r/1465408414-13698-1-git-send-email-ben.dooks@codethink.co.uk
Signed-off-by: Jason Cooper <jason@lakedaemon.net>