Include Kotlin directories in the Dackka plugin (#4847)

Author: mrober

buildSrc/src/main/java/com/google/firebase/gradle/plugins/DackkaPlugin.kt

@@ -174,7 +174,9 @@ abstract class DackkaPlugin : Plugin<Project> {
             val classpath =
               compileConfiguration.jars + project.javadocConfig.jars + project.files(bootClasspath)
 
-            val sourceDirectories = sourceSets.flatMap { it.javaDirectories }
+            val sourceDirectories =
+              sourceSets.flatMap { it.javaDirectories } +
+                sourceSets.flatMap { it.kotlinDirectories }
 
             val packageLists = fetchPackageLists(project)
             val excludedFiles = projectSpecificSuppressedFiles(project)