[NOP] Correct package name in core's xstream package-info.java

Change-Id: I7e498172142ccb2bce8fb0c63e0f2644be1dde16
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1421
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>