Revert "Revert "Add dummy changelog entry for 3.5.1.""

This reverts commit 9a087703.

But we want it in stable after merging master.

debian/changelog

+tails (3.5.1) UNRELEASED; urgency=medium
+
+  * Dummy entry for potential emergency release.
+
+ -- Tails developers <tails@boum.org>  Tue, 23 Jan 2018 01:33:25 +0100
+
 tails (3.5) unstable; urgency=medium
 
   * Security fixes