json: backward compatibilities

* ext/json/generator/generator.c (JSON_Generator_State_type): add
  #ifdef for backward compatibility.
* ext/json/parser/parser.rl (JSON_Parser_type): ditto.
* ext/json/generator/generator.h (ZALLOC): add fallback definition.
* ext/json/parser/parser.h (ZALLOC): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49038 b2dd03c8-39d4-4d8f-98ff-823fe69b080e