package/mender-artifact: needs host-xz

host-xz (for lzma) is needed since version 3.0.0 and
https://github.com/mendersoftware/mender-artifact/commit/8f1b79b03fded3a923d948878905ac5fefe59893

Fixes:
 - http://autobuild.buildroot.org/results/c7cecf8946e9d4b4ae118b5f7597638c70decd3f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>