Set classpath on Painless apiJavadoc task

so it also works on JDK 9

modules/lang-painless/build.gradle

@@ -45,6 +45,7 @@ integTestCluster {
  * Painless plugin */
 task apiJavadoc(type: Javadoc) {
   source = sourceSets.main.allJava
+  classpath = sourceSets.main.runtimeClasspath
   include '**/org/elasticsearch/painless/api/'
   destinationDir = new File(docsDir, 'apiJavadoc')
 }