genirq: Export irq_gc_noop()

In order to build drivers/irqchip/irq-bcm7120-l2.c as a module which
references irq_gc_noop(), we need to export it towards modules.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211020184859.2705451-10-f.fainelli@gmail.com