Create spring-boot-antlib module

Create a new spring-boot-antlib module which allows Apache Ant users
to easily create executable jars.

Fixes gh-3339

spring-boot-tools/pom.xml

@@ -25,5 +25,6 @@
 		<module>spring-boot-loader-tools</module>
 		<module>spring-boot-maven-plugin</module>
 		<module>spring-boot-gradle-plugin</module>
+		<module>spring-boot-antlib</module>
 	</modules>
 </project>