I figured it out myself. I moved the file to src/main/application and it was included in the EAR.
Just for record.
The default resource directory for an EAR is src/main/application, but this can be changed througth the earSourceDirectory property of the maven-ear-plugin like the next example.
<!-- Tell Maven we are using Java EE 6 -->
<!-- Use Java EE ear libraries as needed. Java EE ear libraries
are in easy way to package any libraries needed in the ear, and automatically
have any modules (EJB-JARs and WARs) use them -->