[MINOR] Fix Scala 2.12 build

## What changes were proposed in this pull request?

[SPARK-25095](ad45299) introduced `ambiguous reference to overloaded definition`

```
[error] /Users/d_tsai/dev/apache-spark/core/src/main/scala/org/apache/spark/api/python/PythonRunner.scala:242: ambiguous reference to overloaded definition,
[error] both method addTaskCompletionListener in class TaskContext of type [U](f: org.apache.spark.TaskContext => U)org.apache.spark.TaskContext
[error] and  method addTaskCompletionListener in class TaskContext of type (listener: org.apache.spark.util.TaskCompletionListener)org.apache.spark.TaskContext
[error] match argument types (org.apache.spark.TaskContext => Unit)
[error]           context.addTaskCompletionListener(_ => server.close())
[error]                   ^
[error] one error found
[error] Compile failed at Aug 24, 2018 1:56:06 PM [31.582s]
```
which fails the Scala 2.12 branch build.

## How was this patch tested?

Existing tests

Closes #22229 from dbtsai/fix-2.12-build.

Authored-by: DB Tsai <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>

Author: dbtsai

core/src/main/scala/org/apache/spark/api/python/PythonRunner.scala

@@ -239,7 +239,7 @@ private[spark] abstract class BasePythonRunner[IN, OUT](
         }
         // Close ServerSocket on task completion.
         serverSocket.foreach { server =>
-          context.addTaskCompletionListener(_ => server.close())
+          context.addTaskCompletionListener[Unit](_ => server.close())
         }
         val boundPort: Int = serverSocket.map(_.getLocalPort).getOrElse(0)
         if (boundPort == -1) {