[SPARK-3347] [yarn] Fix yarn-alpha compilation.

Missing import. Oops.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes apache#2236 from vanzin/SPARK-3347 and squashes the following commits:

594fc39 [Marcelo Vanzin] [SPARK-3347] [yarn] Fix yarn-alpha compilation.

yarn/alpha/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -32,6 +32,7 @@ import org.apache.hadoop.yarn.ipc.YarnRPC
 import org.apache.hadoop.yarn.util.{Apps, Records}
 
 import org.apache.spark.{Logging, SparkConf}
+import org.apache.spark.deploy.SparkHadoopUtil
 
 /**
  * Version of [[org.apache.spark.deploy.yarn.ClientBase]] tailored to YARN's alpha API.