build: switch source archive to xz compression

Change-Id: I33870abd65bc28baf0053daa5ddb0a4a592f928c
diff --git a/wscript b/wscript
index 8a4d2d4..e6a986e 100644
--- a/wscript
+++ b/wscript
@@ -89,3 +89,9 @@
             name='systemd-units',
             source='systemd/repo-ng.service.in',
             target='systemd/repo-ng.service')
+
+def dist(ctx):
+    ctx.algo = 'tar.xz'
+
+def distcheck(ctx):
+    ctx.algo = 'tar.xz'