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>
diff --git a/kernel/utsname.c b/kernel/utsname.c
index 816d7b2..64d398f 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -14,6 +14,7 @@
 #include <linux/utsname.h>
 #include <linux/version.h>
 #include <linux/err.h>
+#include <linux/slab.h>
 
 /*
  * Clone a new ns copying an original utsname, setting refcount to 1