State.Host: use set for AllowedHostTransitions

The intended use of this is as a set of allowed elements.  Express
intent with `set` instead of `array`, even though these end up on dbus
in the same data type.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0c2f55daa704f578ed1bb45a93ad81b4c57b90fc
diff --git a/yaml/xyz/openbmc_project/State/Host.interface.yaml b/yaml/xyz/openbmc_project/State/Host.interface.yaml
index 6d3f4a4..95f3b19 100644
--- a/yaml/xyz/openbmc_project/State/Host.interface.yaml
+++ b/yaml/xyz/openbmc_project/State/Host.interface.yaml
@@ -13,7 +13,7 @@
           - xyz.openbmc_project.Common.Error.NotAllowed
 
     - name: AllowedHostTransitions
-      type: array[enum[self.Transition]]
+      type: set[enum[self.Transition]]
       flags:
           - const
       description: >