Add missing fileutils require

On my system, the error was being hidden by the presence of a YARD
rubygems plugin that was providing the require and making things work.

lib/rubygems/source.rb

@@ -179,7 +179,10 @@ class Gem::Source
     local_file = File.join(cache_dir, file_name)
     retried    = false
 
-    FileUtils.mkdir_p cache_dir if update_cache?
+    if update_cache?
+      require "fileutils"
+      FileUtils.mkdir_p cache_dir
+    end
 
     spec_dump = fetcher.cache_update_path spec_path, local_file, update_cache?