debian/changelog: change last release for unreleased to unstable

Otherwise osmo-release.mk overrides contents of last release.

Change-Id: Ife77f6ddd45465b0a5fce80e51e3e64428787b91
diff --git a/debian/changelog b/debian/changelog
index 08636ef..ab33e8e 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,4 +1,4 @@
-openbsc (1.0.0) UNRELEASED; urgency=medium
+openbsc (1.0.0) unstable; urgency=medium
 
   [ Holger Hans Peter Freyther ]
   * Revert "gprs: Use RAND_bytes for p-tmsi"