Switch from include guard to pragma once.

This pragma is so widley supported it is a defacto standard. It has the
advantage over include guards in that a unique macro identifier does not
need to be created and maintained.

Change-Id: Id12f56d9970fdfb63d45c92e428d3dbe44b2ab0e
diff --git a/inc/hf/std.h b/inc/hf/std.h
index d7cc124..e17f155 100644
--- a/inc/hf/std.h
+++ b/inc/hf/std.h
@@ -1,5 +1,4 @@
-#ifndef _STD_H
-#define _STD_H
+#pragma once
 
 #include <stddef.h>
 #include <stdint.h>
@@ -54,5 +53,3 @@
 #error "Unsupported byte order"
 
 #endif
-
-#endif /* STD_H */