Add .tar.xz, .txz, and .orig.tar.xz as known extensions to dl_cleanup.py.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35645 3c298f89-4303-0410-b956-a3cf2f4a3e73