CHANGELOG.md: Mention stack-protector feature

Stack protector is meant to be enabled on all architectures, but
currently it is tested (and enabled) only on ARM, so mention it in ARM
section.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -19,6 +19,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
      capability usage is not yet supported on PVH dom0).
    - Smoke tests for the FreeBSD Xen builds in Cirrus CI.
 
+ - On Arm:
+    - Ability to enable stack protector
+
 ### Removed
 
 ## [4.20.0](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.20.0) - 2025-03-05