Adding proguard.cfg to library-projects-aar-abs-example

This should have been committed with https://github.com/jayway/maven-android-plugin-samples/commit/8799c4556097af5b4c70c493c06b508a05f2b445

Without it the librayprojects-aar-abs-example project ignores the instruction to use Proguard.

NB We should consider breaking the build if proguard=true and the configured proguard.cfg file can't be found.