Fixed install error with rdoc.gemspec

"lib/rdoc" is only workds on rdoc repository structure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59848 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/rdoc.gemspec

@@ -1,4 +1,10 @@
-require_relative "lib/rdoc"
+# -*- encoding: utf-8 -*-
+begin
+  require_relative "lib/rdoc"
+rescue LoadError
+  # for Ruby repository
+  require_relative "../rdoc"
+end
 
 Gem::Specification.new do |s|
   s.name = "rdoc"