debian/changelog: Set previous versions to unstable

Otherwise make rlease overwrittes the latest UNRELEASED one.

Change-Id: Ic6a9b1d6c7724b15c69e780d562007b22af141e6
diff --git a/debian/changelog b/debian/changelog
index 20354b0..457e269 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,10 +1,10 @@
-libasn1c (0.9.28) UNRELEASED; urgency=low
+libasn1c (0.9.28) unstable; urgency=low
 
   * Initial release.
 
  -- Harald Welte <laforge@gnumonks.org>  Fri, 27 Oct 2017 22:08:03 +0000
 
-libasn1c (0.1.0) UNRELEASED; urgency=low
+libasn1c (0.1.0) unstable; urgency=low
 
   * Initial release.