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.

Change-Id: I86abfcf31dadbb11c2b5db6528bc61cf3d8e87a0
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
Git-commit: 34a866bd45d69754da1979e83a37bec6defc6295
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Vijay Navnath Kamble <vkambl@codeaurora.org>