Add changelog entry

Author: rlazo

firebase-crashlytics/CHANGELOG.md

@@ -1,5 +1,5 @@
 # Unreleased
-
+* [fixed] Execute failure listener outside the main thread  [#6535]
 
 # 19.2.1
 * [changed] Updated protobuf dependency to `3.25.5` to fix
@@ -634,4 +634,3 @@ The following release notes describe changes in the new SDK.
   from your `AndroidManifest.xml` file.
 * [removed] The `fabric.properties` and `crashlytics.properties` files are no
   longer supported. Remove them from your app.
-