Update watch file

Upstream switched to xz

debian/watch

@@ -1,4 +1,4 @@
 
 version=3
 https://github.com/latchset/clevis/releases \
-    /latchset/clevis/releases/download/[^/]+/clevis-([0-9][.0-9]*)\.tar\.bz2
+    /latchset/clevis/releases/download/[^/]+/clevis-([0-9][.0-9]*)\.tar\.xz