Add missing @ConfigurationProperties on sample

Required after it became mandatory in Spring Boot:

3740c817d3
https://github.com/spring-projects/spring-boot/issues/3460

spring-cloud-stream-samples/source/src/main/java/demo/TimeSourceOptionsMetadata.java

@@ -19,12 +19,15 @@ package demo;
 import javax.validation.constraints.Min;
 import javax.validation.constraints.Pattern;
 
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
 /**
  * Describes options to the {@code time} source module.
  *
  * @author Eric Bottard
  * @author Gary Russell
  */
+@ConfigurationProperties
 public class TimeSourceOptionsMetadata {
 
 	/**