genirq/irq_sim: Explicitly include slab.h

kfree() is used in the irq_sim code but slab.h is pulled in indirectly via
irq.h. Include it explicitly.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/20180304121018.640-2-brgl@bgdev.pl