test_rdoc_generator_darkfish.rb: ignore nlink is not provided

* test/rdoc/test_rdoc_generator_darkfish.rb (assert_hard_link): makes
  no sense on platforms where link count is not provided.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e