Missing changelog entry

(cherry picked from commit 53fca645)

Doc/changelog.docu

@@ -38,6 +38,7 @@
 <ul>
 <li>Added clean_keep_reservation() (remove all elements from the mesh but keeps the properties. In contrast to clean() the memory used for the elements will remain allocated</li>
 <li>Reduced number of include statements to speedup compilation</li>
+<li>Removed unused member variable next_cache_count_ member of PolyConnectivity and made it local</li>
 </ul>
 
 <b>General</b>