Add @Configuration to WebMvcAutoConfigurationAdapter

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java

@@ -125,6 +125,7 @@ public class WebMvcAutoConfiguration {
 	// Defined as a nested config to ensure WebMvcConfigurerAdapter it not read when not
 	// on the classpath
 	@EnableWebMvc
+	@Configuration
 	public static class WebMvcAutoConfigurationAdapter extends WebMvcConfigurerAdapter {
 
 		private static Log logger = LogFactory.getLog(WebMvcConfigurerAdapter.class);