CHANGELOG: add entry for removed c-stubdom

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Teddy Astie <teddy.astie@vates.tech>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -17,6 +17,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - The kexec "v1" interface, which was declared obsolete in Xen 4.4 (2013).
      The only known user was the classic-xen fork of Linux.  This does not
      affect Xen kexec support in the kexec-tools package.
+   - The example stubdom "c-stubdom" has been removed.
 
 ## [4.22.0 UNRELEASED](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=staging) - TBD