Fixed import.

Author: rxin

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

@@ -21,12 +21,12 @@ import java.lang.management.ManagementFactory
 import java.lang.reflect.{Field, Modifier}
 import java.util.{IdentityHashMap, Random}
 import java.util.concurrent.ConcurrentHashMap
-import org.apache.spark.annotation.DeveloperApi
 
 import scala.collection.mutable.ArrayBuffer
 import scala.runtime.ScalaRunTime
 
 import org.apache.spark.Logging
+import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.util.collection.OpenHashSet