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/backward/rubyio.h

@@ -1,6 +1,18 @@
+#ifndef RUBY_BACKWARD_RUBYIO_H                       /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY_BACKWARD_RUBYIO_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
 #if   defined __GNUC__
 #warning use "ruby/io.h" instead of "rubyio.h"
 #elif defined _MSC_VER
 #pragma message("warning: use \"ruby/io.h\" instead of \"rubyio.h\"")
 #endif
 #include "ruby/io.h"
+
+#endif /* RUBY_BACKWARD_RUBYIO_H */