Add dependency needed by javadoc generation on JDK 11

spring-boot-project/spring-boot-docs/pom.xml

@@ -658,6 +658,11 @@
 			<artifactId>jboss-logging</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.jetbrains.kotlin</groupId>
+			<artifactId>kotlin-reflect</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.jetbrains.kotlin</groupId>
 			<artifactId>kotlin-stdlib</artifactId>