Fix a memory leak

Before ROOT allocated the member arrays of layout_ of the newObj, that
are then overwritten by ROOTReadStreamer without deallocation. Setting
layout_ as the target as well tells ROOT to not to do those allocations.

DataFormats/PortableTestObjects/src/alpaka/classes_serial_def.xml

@@ -5,7 +5,7 @@
     targetClass="portabletest::TestHostCollection"
     version="[1-]"
     source="portabletest::TestSoA layout_;"
-    target="buffer_"
+    target="buffer_,layout_"
     embed="false">
   <![CDATA[
     portabletest::TestHostCollection::ROOTReadStreamer(newObj, onfile.layout_);