Added gemspec file to gem package

The original gemspec need to install rdoc as bundled gems at the
official Ruby package.

Fix https://bugs.ruby-lang.org/issues/21312

Author: hsbt

rdoc.gemspec

@@ -38,6 +38,7 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   # for ruby core repository. It was generated by
   # `git ls-files -z`.split("\x0").each {|f| puts "    #{f.dump}," unless f.start_with?(*%W[test/ spec/ features/ .]) }
   non_lib_files = [
+    "rdoc.gemspec",
     "CONTRIBUTING.rdoc",
     "CVE-2013-0256.rdoc",
     "ExampleMarkdown.md",