Make hash_encoded private on JRuby

Argon2id::Password.hash_encoded is defined as a private singleton method
in the C extension so we use private_class_method to do the same for
JRuby.

lib/argon2id/extension.rb

@@ -35,6 +35,8 @@ def self.hash_encoded(t_cost, m_cost, parallelism, pwd, salt, hashlen)
         raise Error, e.message
       end
 
+      private_class_method :hash_encoded
+
       private
 
       def verify(pwd)
@@ -65,4 +67,3 @@ def verify(pwd)
     require "argon2id/argon2id"
   end
 end
-