genirq: Let GENERIC_IRQ_IPI select IRQ_DOMAIN_HIERARCHY

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

kernel/irq/Kconfig

@@ -82,6 +82,7 @@ config IRQ_FASTEOI_HIERARCHY_HANDLERS
 # Generic IRQ IPI support
 config GENERIC_IRQ_IPI
 	bool
+	select IRQ_DOMAIN_HIERARCHY
 
 # Generic MSI interrupt support
 config GENERIC_MSI_IRQ