Add private[spark] to EventLoop

Author: zsxwing

core/src/main/scala/org/apache/spark/util/EventLoop.scala

@@ -27,7 +27,7 @@ import org.apache.spark.Logging
  * An event loop to receive events from the caller and process all events in the event thread. It
  * will start an exclusive event thread to process all events.
  */
-abstract class EventLoop[E](name: String) extends Logging {
+private[spark] abstract class EventLoop[E](name: String) extends Logging {
 
   private val eventQueue: BlockingQueue[E] = new LinkedBlockingDeque[E]()