fileutils.rb: do not make root

	* lib/fileutils.rb (FileUtils#mkdir_p): no need to make root
	  directory which should be exist and cannot be made with mkdir
	  recent Cygwin can make a directory contains a colon.
	  [Bug #13214]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58631 b2dd03c8-39d4-4d8f-98ff-823fe69b080e