fix compile error of streaming project

explicit return type for implicit function

Author: James Z.M. Gao

streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala

@@ -736,7 +736,7 @@ class JavaPairDStream[K, V](val dstream: DStream[(K, V)])(
 }
 
 object JavaPairDStream {
-  implicit def fromPairDStream[K: ClassTag, V: ClassTag](dstream: DStream[(K, V)]) = {
+  implicit def fromPairDStream[K: ClassTag, V: ClassTag](dstream: DStream[(K, V)]): JavaPairDStream[K, V] = {
     new JavaPairDStream[K, V](dstream)
   }