Move headers under the hf directory.

This helps distinguish local headers from the more official headers.

Change-Id: I535c1b44081b9d60ba63666cd1ad514aaaf2c68d
diff --git a/src/std.c b/src/std.c
index 9ea1591..6d26bef 100644
--- a/src/std.c
+++ b/src/std.c
@@ -1,4 +1,4 @@
-#include "std.h"
+#include "hf/std.h"
 
 void *memset(void *s, int c, size_t n)
 {