Mark the mutex_owned_per_thread? method as private.

Author: ioquatix

lib/concurrent-ruby/concurrent/atomic/lock_local_var.rb

@@ -2,6 +2,7 @@
 require_relative 'thread_local_var'
 
 module Concurrent
+  # @!visibility private
   def self.mutex_owned_per_thread?
     mutex = Mutex.new