SYSFS: Explicitly include required header file slab.h.

After an experimental deletion of the unnecessary inclusion of
<linux/slab.h> from the header file <linux/percpu.h>, the following
files under fs/sysfs were exposed as needing to explicitly include
<linux/slab.h>.
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/dir.c

@@ -20,6 +20,7 @@
 #include <linux/idr.h>
 #include <linux/completion.h>
 #include <linux/mutex.h>
+#include <linux/slab.h>
 #include "sysfs.h"
 
 DEFINE_MUTEX(sysfs_mutex);

fs/sysfs/file.c

@@ -13,6 +13,7 @@
 #include <linux/module.h>
 #include <linux/kobject.h>
 #include <linux/kallsyms.h>
+#include <linux/slab.h>
 #include <linux/namei.h>
 #include <linux/poll.h>
 #include <linux/list.h>