Make MutexAtomicBoolean#ns_make_value private

This makes the implementation more similar to MutexAtomicFixnum.

Author: qnet-herwin

lib/concurrent/atomic/mutex_atomic_boolean.rb

@@ -50,6 +50,8 @@ def ns_initialize(initial)
       @value = !!initial
     end
 
+    private
+
     # @!visibility private
     def ns_make_value(value)
       old = @value