[HOTFIX] Fix Scala 2.11 compilation

by explicitly marking annotated parameters as vals (SI-8813).

Caused by #10835.

Author: Andrew Or <andrew@databricks.com>

Closes #10955 from andrewor14/fix-scala211.

core/src/main/scala/org/apache/spark/Accumulable.scala

@@ -57,7 +57,8 @@ import org.apache.spark.util.Utils
  */
 class Accumulable[R, T] private (
     val id: Long,
-    @transient initialValue: R,
+    // SI-8813: This must explicitly be a private val, or else scala 2.11 doesn't compile
+    @transient private val initialValue: R,
     param: AccumulableParam[R, T],
     val name: Option[String],
     internal: Boolean,