Use alternative syntax to concatenate lists.

Change-Id: Ie74b2e3a733b329e678ade27dffe2124083bf072
diff --git a/vars/functions.groovy b/vars/functions.groovy
index 7419b34..6e7180f 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -67,7 +67,7 @@
         }
         
         def filenames = (packages + sources).each{entry -> entry.path}
-        packageFilenames += filenames
+        packageFilenames = packageFilenames + filenames
     }