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

include/ruby/3/xmalloc.h

@@ -1,4 +1,6 @@
-/**                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY3_XMALLOC_H                              /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY3_XMALLOC_H
+/**
  * @file
  * @author     Ruby developers <ruby-core@ruby-lang.org>
  * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
@@ -18,8 +20,6 @@
  *             extension libraries. They could be written in C++98.
  * @brief      Declares ::ruby_xmalloc().
  */
-#ifndef  RUBY3_XMALLOC_H
-#define  RUBY3_XMALLOC_H
 #include "ruby/3/config.h"
 
 #ifdef STDC_HEADERS