genirq: Add stub for set_handle_irq() when !GENERIC_IRQ_MULTI_HANDLER

In order to avoid compilation errors when a driver references set_handle_irq(),
but that the architecture doesn't select GENERIC_IRQ_MULTI_HANDLER,
add a stub function that will just WARN_ON_ONCE() if ever used.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[maz: commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200924071754.4509-2-thunder.leizhen@huawei.com