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

transcode_data.h

@@ -1,3 +1,5 @@
+#ifndef RUBY_TRANSCODE_DATA_H
+#define RUBY_TRANSCODE_DATA_H 1
 /**********************************************************************
 
   transcode_data.h -
@@ -11,9 +13,6 @@
 
 #include "ruby/ruby.h"
 
-#ifndef RUBY_TRANSCODE_DATA_H
-#define RUBY_TRANSCODE_DATA_H 1
-
 RUBY_SYMBOL_EXPORT_BEGIN
 
 #define WORDINDEX_SHIFT_BITS 2