Add missing changelog entry

This entry was meant to be integrated in commit
c824c49b3fbbd61626031eeb10ae37782a994154.

debian/changelog

@@ -51,6 +51,8 @@ dpkg (1.16.0) UNRELEASED; urgency=low
   * Fix dpkg-source to not list Debian packaging files as modified
     upstream files in Format "1.0" when unpacking to a non-standard
     directory.
+  * Apply patch from Colin Watson to let dpkg-buildflags return -O3
+    instead of -O2 when building ppc64 packages on Ubuntu. Closes: #612472
 
   [ Jonathan Nieder ]
   * Remove support for use of synchronous sync(2), due to its pernicious