add #include guard hack

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

vm_debug.h

@@ -1,3 +1,5 @@
+#ifndef RUBY_DEBUG_H
+#define RUBY_DEBUG_H
 /**********************************************************************
 
   vm_debug.h - YARV Debug function interface
@@ -9,9 +11,6 @@
 
 **********************************************************************/
 
-#ifndef RUBY_DEBUG_H
-#define RUBY_DEBUG_H
-
 #include "ruby/ruby.h"
 #include "node.h"