Remove kernel headers from rbtree.h

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/rbtree.h b/rbtree.h
index 344bc34..879efa8 100644
--- a/rbtree.h
+++ b/rbtree.h
@@ -94,8 +94,7 @@
 #ifndef	_LINUX_RBTREE_H
 #define	_LINUX_RBTREE_H
 
-#include <linux/kernel.h>
-#include <linux/stddef.h>
+#include <stdlib.h>
 
 struct rb_node
 {