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.ukSigned-off-by: Jason Cooper <jason@lakedaemon.net>

drivers/irqchip/irq-gic-v2m.c

@@ -24,6 +24,7 @@
 #include <linux/of_pci.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/irqchip/arm-gic.h>
 
 /*
 * MSI_TYPER: