Add explicitly require for Dir.mktmpdir

test/io/wait/test_io_wait_uncommon.rb

@@ -19,6 +19,7 @@ class TestIOWaitUncommon < Test::Unit::TestCase
 
   def test_fifo_wait
     skip 'no mkfifo' unless File.respond_to?(:mkfifo) && IO.const_defined?(:NONBLOCK)
+    require 'tmpdir'
     Dir.mktmpdir('rubytest-fifo') do |dir|
       fifo = "#{dir}/fifo"
       assert_equal 0, File.mkfifo(fifo)