[MonologBundle] Make monolog.logger private

src/Symfony/Bundle/MonologBundle/Resources/config/monolog.xml

@@ -17,7 +17,7 @@
     </parameters>
 
     <services>
-        <service id="monolog.logger" parent="monolog.logger_prototype">
+        <service id="monolog.logger" parent="monolog.logger_prototype" public="false">
             <argument index="0">app</argument>
         </service>
         <service id="logger" alias="monolog.logger" />