[rubygems/rubygems] Add missing require "fileutils" in lib/rubygems/installer.rb

ruby/rubygems@9a9d0e423e

Author: MSP-Greg

lib/rubygems/installer.rb

@@ -953,6 +953,7 @@ def write_cache_file
   end
 
   def ensure_writable_dir(dir) # :nodoc:
+    require "fileutils"
     FileUtils.mkdir_p dir, mode: options[:dir_mode] && 0o755
 
     raise Gem::FilePermissionError.new(dir) unless File.writable? dir