[FLINK-19001] Set scope provided.

flink-streaming-java should always be with scope provided. It wasn't
explicitly needed before, because this artifact was not part of any
jar-with-dependencies.
diff --git a/statefun-flink/statefun-flink-core/pom.xml b/statefun-flink/statefun-flink-core/pom.xml
index e9babcc..b67aa02 100644
--- a/statefun-flink/statefun-flink-core/pom.xml
+++ b/statefun-flink/statefun-flink-core/pom.xml
@@ -56,6 +56,7 @@
         <dependency>
             <groupId>org.apache.flink</groupId>
             <artifactId>flink-streaming-java_${scala.binary.version}</artifactId>
+            <scope>provided</scope>
         </dependency>
 
         <!-- 3rd party -->