kernel.h: add header guard

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>

include/linux/kernel.h

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index b14e25b..b124a45 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -1,3 +1,6 @@
+#ifndef _LINUX_KERNEL_H
+#define _LINUX_KERNEL_H
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
@@ -34,3 +37,5 @@
 while (0)
 
 #define __used		__attribute__((__used__))
+
+#endif /* _LINUX_KERNEL_H */