genirq: Let GENERIC_IRQ_IPI select IRQ_DOMAIN_HIERARCHY

[ Upstream commit 151a535171be6ff824a0a3875553ea38570f4c05 ]

kernel/irq/ipi.c otherwise fails to compile if nothing else
selects it.

Fixes: 379b6564 ("genirq: Add GENERIC_IRQ_IPI Kconfig symbol")
Reported-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201015101222.GA32747@amd


Signed-off-by: Sasha Levin <sashal@kernel.org>