src/include: Add guards on all header files

Change-Id: I2d7d4e0b25f2cf3eef2040f89d5ebc711909cdd7
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20734
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/include/sys/types.h

@@ -1 +1,6 @@
+#ifndef __SYS_TYPES_H__
+#define __SYS_TYPES_H__
+
 #include "../types.h"
+
+#endif /* __SYS_TYPES_H__ */