Add missing changelog entry for 0.23.

debian/changelog

 tails (0.23) unstable; urgency=medium
 
+  * Security fixes
+    - Upgrade the web browser to 24.4.0esr-0+tails1~bpo60+1
+      (Firefox 24.4.0esr + Iceweasel patches + Torbrowser patches).
+
   * Major new features
     - Spoof the network interfaces' MAC address by default (Closes: #5421),
       as designed on https://tails.boum.org/contribute/design/MAC_address/.