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

kernel/irq/irq_sim.c

@@ -7,6 +7,7 @@
  * option) any later version.
  */
 
+#include <linux/slab.h>
 #include <linux/irq_sim.h>
 #include <linux/irq.h>