Fix Ant builds following Maven updates

Update `spring-boot-sample-ant` so that it works with the CI friendly
Maven POMs.

See gh-9316

spring-boot-samples/spring-boot-sample-ant/pom.xml

@@ -21,6 +21,10 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-loader</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>