[SPARK-21898][ML][FOLLOWUP] Fix Scala 2.12 build.

## What changes were proposed in this pull request?

This is a follow-up pr of apache#19108 which broke Scala 2.12 build.

```
[error] /Users/ueshin/workspace/apache-spark/spark/mllib/src/main/scala/org/apache/spark/ml/stat/KolmogorovSmirnovTest.scala:86: overloaded method value test with alternatives:
[error]   (dataset: org.apache.spark.sql.DataFrame,sampleCol: String,cdf: org.apache.spark.api.java.function.Function[java.lang.Double,java.lang.Double])org.apache.spark.sql.DataFrame <and>
[error]   (dataset: org.apache.spark.sql.DataFrame,sampleCol: String,cdf: scala.Double => scala.Double)org.apache.spark.sql.DataFrame
[error]  cannot be applied to (org.apache.spark.sql.DataFrame, String, scala.Double => java.lang.Double)
[error]     test(dataset, sampleCol, (x: Double) => cdf.call(x))
[error]     ^
[error] one error found
```

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <[email protected]>

Closes apache#20994 from ueshin/issues/SPARK-21898/fix_scala-2.12.

Author: ueshin

mllib/src/main/scala/org/apache/spark/ml/stat/KolmogorovSmirnovTest.scala

@@ -83,7 +83,8 @@ object KolmogorovSmirnovTest {
   @Since("2.4.0")
   def test(dataset: DataFrame, sampleCol: String,
     cdf: Function[java.lang.Double, java.lang.Double]): DataFrame = {
-    test(dataset, sampleCol, (x: Double) => cdf.call(x))
+    val f: Double => Double = x => cdf.call(x)
+    test(dataset, sampleCol, f)
   }
 
   /**