set distribution=unstable for upload to Debian

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
diff --git a/debian/changelog b/debian/changelog
index 289b18e..d259076 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,4 +1,4 @@
-rt-tests (0.39-2) UNRELEASED; urgency=low
+rt-tests (0.39-2) unstable; urgency=low
 
   * add VCS URL to control
   * conflict with xenomai-runtime (Closes: #529783)