[ruby/json] Add lib/json/ext/generator/state.rb to the gemspec

* Otherwise the gem always uses the pure-Ruby backend
  as it's missing that file and rescuing the LoadError.

https://github.com/ruby/json/commit/1e2809b0b0

ext/json/json.gemspec

@@ -50,6 +50,7 @@ Gem::Specification.new do |s|
     "lib/json/add/time.rb",
     "lib/json/common.rb",
     "lib/json/ext.rb",
+    "lib/json/ext/generator/state.rb",
     "lib/json/generic_object.rb",
     "lib/json/pure.rb",
     "lib/json/pure/generator.rb",