I am getting the following error while obfuscating my application.
How can i define the output jar in my build.xml?

```
 [java] Shrinking...
 [java] java.io.IOException: The output jar is empty. Did you specify the proper '-keep' options?
 [java]     at proguard.shrink.Shrinker.execute(Shrinker.java:148)
 [java]     at proguard.ProGuard.shrink(ProGuard.java:294)
 [java]     at proguard.ProGuard.execute(ProGuard.java:100)
 [java]     at proguard.ProGuard.main(ProGuard.java:499)

```