Add missing require "concurrent/set.rb".

Author: ioquatix

spec/concurrent/atomic/reentrant_read_write_lock_spec.rb

@@ -2,6 +2,7 @@
 require 'concurrent/atomic/reentrant_read_write_lock'
 require 'concurrent/atomic/count_down_latch'
 require 'concurrent/atomic/atomic_boolean'
+require 'concurrent/set'
 require 'timeout'
 require 'fiber'