MAINTAINERS: add Xen Domain-0 snippet to Xen Platform

The snippet for Zephyr as Xen Domain-0 was missing in the
Xen Platform section of MAINTAINERS.yml.

Add the missing entry to ensure that future pull requests
for this component are correctly tracked and assigned.

Signed-off-by: Dmytro Firsov <[email protected]>

Author: firscity

MAINTAINERS.yml

@@ -5912,6 +5912,7 @@ Xen Platform:
     - soc/xen/
     - boards/xen/
     - dts/bindings/xen/
+    - snippets/xen_dom0/
   labels:
     - "area: Xen Platform"