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>