build: switch source archive to xz compression

Change-Id: Id48d81e5b91aad8b2b69925e2c8ed3506e7e17d9
diff --git a/wscript b/wscript
index ad13479..a4f9500 100644
--- a/wscript
+++ b/wscript
@@ -119,3 +119,9 @@
             name='systemd-units',
             source='systemd/ndncert-ca.service.in',
             target='systemd/ndncert-ca.service')
+
+def dist(ctx):
+    ctx.algo = 'tar.xz'
+
+def distcheck(ctx):
+    ctx.algo = 'tar.xz'