debian/changelog: Set latest release as unstable

It was left as UNRELEASED but it was actually released (there's a tag on
it). Otherwise osmo-release.mk is fooled.

Change-Id: Iedd8311303274dd3be88a11314a61fbdd511679d
diff --git a/debian/changelog b/debian/changelog
index 69b49ae..851dee5 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,4 +1,4 @@
-libgtpnl (1.1.0) UNRELEASED; urgency=medium
+libgtpnl (1.1.0) unstable; urgency=medium
 
   * Add genl_socket_close() function
   * Add contrib/jenkins.sh for jenkins build verification