package/xmlstarlet: Fix static build

If libxml2 depends on xz (liblzma) static linking is broken.

Fixes
http://autobuild.buildroot.net/results/df3/df3f5fa770833c8c4344ee0bc67f45aa578ce542/
http://autobuild.buildroot.net/results/070/07021c8bd6e2031c2717fc58e2d3ff30c412fa90/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>