Add missing changelog entry

Author: badboy

CHANGELOG.md

@@ -6,6 +6,8 @@
   * Updated to `glean_parser` v18.2.0 ([#3356](https://github.com/mozilla/glean/issues/3356))
 * Python
   * Source wheel builds now build the SDK upon install correctly ([#3359](https://github.com/mozilla/glean/pull/3359))
+* iOS
+  * Annotate the `PingUploadScheduler` as unavailable for iOS App extensions ([, as app extensions run in a restricted environment without access to UIApplication.shared.#3355](https://github.com/mozilla/glean/pull/3355))
 
 # v66.2.0 (2025-12-09)