devtmpfs: add base.h include

This fixes a sparse warning, and is a good idea given that the
devtmpfs_init() prototype is in this file.

Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/devtmpfs.c

···
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/kthread.h>
+#include "base.h"
 
 static struct task_struct *thread;