debian/changelog: document removal of 48-systemd-workaround.diff patch

debian/changelog

@@ -35,6 +35,7 @@ mumble (1.3.0~git20190114.9fcc588+dfsg-1) UNRELEASED; urgency=medium
     - Remove 43-initialize-SSL.diff (similar fix incorporated upstream)
     - Update 44-add-speechd-header.diff for new upstream snapshot
     - Update 46-var-run-to-run.diff for new upstream snapshot
+    - Remove 48-systemd-workaround.diff (incorporated upstream)
   * debian/upstream:
     - Remove signing-key.asc due to having to repack upstream tarball
       which will cause the PGP signature check to fail