kernel: explicitly include required header files under kernel/

Following an experimental deletion of the unnecessary directive

 #include <linux/slab.h>

from the header file <linux/percpu.h>, these files under kernel/ were exposed
as needing to include one of <linux/slab.h> or <linux/gfp.h>, so explicit
includes were added where necessary.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/irq/manage.c

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 438a014..46e4ad1 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/random.h>
 #include <linux/interrupt.h>
+#include <linux/slab.h>
 
 #include "internals.h"