irqchip/gic-v4: Add forward definition of struct irq_domain_ops

In some randconfig scenarios, including arm-gic-v4.h results
in a spurious wawrning about the $SUBJECT structure not being
defined. Adding a forward definition keeps it quiet.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

include/linux/irqchip/arm-gic-v4.h

@@ -109,6 +109,7 @@ int its_get_vlpi(int irq, struct its_vlpi_map *map);
 int its_unmap_vlpi(int irq);
 int its_prop_update_vlpi(int irq, u8 config, bool inv);
 
+struct irq_domain_ops;
 int its_init_v4(struct irq_domain *domain, const struct irq_domain_ops *ops);
 
 #endif