[MINOR] Added import to fix compilation

## What changes were proposed in this pull request?

Two back to PRs implicitly conflicted by one PR removing an existing import that the other PR needed. This did not cause explicit conflict as the import already existed, but not used.

https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Compile/job/spark-master-compile-maven-hadoop-2.7/8226/consoleFull

```
[info] Compiling 342 Scala sources and 97 Java sources to /home/jenkins/workspace/spark-master-compile-maven-hadoop-2.7/sql/core/target/scala-2.11/classes...
[warn] /home/jenkins/workspace/spark-master-compile-maven-hadoop-2.7/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala:128: value ENABLE_JOB_SUMMARY in object ParquetOutputFormat is deprecated: see corresponding Javadoc for more information.
[warn]       && conf.get(ParquetOutputFormat.ENABLE_JOB_SUMMARY) == null) {
[warn]                                       ^
[error] /home/jenkins/workspace/spark-master-compile-maven-hadoop-2.7/sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/statefulOperators.scala:95: value asJava is not a member of scala.collection.immutable.Map[String,Long]
[error]       new java.util.HashMap(customMetrics.mapValues(long2Long).asJava)
[error]                                                                ^
[warn] one warning found
[error] one error found
[error] Compile failed at Aug 21, 2018 4:04:35 PM [12.827s]
```

## How was this patch tested?
It compiles!

Closes #22175 from tdas/fix-build.

Authored-by: Tathagata Das <[email protected]>
Signed-off-by: Tathagata Das <[email protected]>

Author: tdas

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/statefulOperators.scala

@@ -20,6 +20,8 @@ package org.apache.spark.sql.execution.streaming
 import java.util.UUID
 import java.util.concurrent.TimeUnit._
 
+import scala.collection.JavaConverters._
+
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.errors._