[SPARK-7726] Add import so Scaladoc doesn't fail.

This is another import needed so Scala 2.11 doc generation doesn't fail.
See SPARK-7726 for more detail. I tested this locally and the 2.11
install goes from failing to succeeding with this patch.

Author: Patrick Wendell <patrick@databricks.com>

Closes #8095 from pwendell/scaladoc.

network/shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/mesos/RegisterDriver.java

@@ -23,6 +23,9 @@ import io.netty.buffer.ByteBuf;
 import org.apache.spark.network.protocol.Encoders;
 import org.apache.spark.network.shuffle.protocol.BlockTransferMessage;
 
+// Needed by ScalaDoc. See SPARK-7726
+import static org.apache.spark.network.shuffle.protocol.BlockTransferMessage.Type;
+
 /**
  * A message sent from the driver to register with the MesosExternalShuffleService.
  */