Fix a bug introduced in cc4d68c68d.

The `alive` variant could sometimes be set preventing the garrison flag
from showing.
Reported by: Gurken Khan
Thread: https://wildfiregames.com/forum/topic/106480-flagless-uxelon/

This was SVN commit r27571.

binaries/data/mods/public/art/actors/structures/britons/wooden_tower.xml

@@ -26,7 +26,7 @@
     </variant>
   </group>
   <group>
-    <variant frequency="1" name="alive"/>
+    <variant frequency="1" name="ungarrisoned"/>
     <variant name="garrisoned">
       <props>
         <prop actor="props/special/common/garrison_flag_celt.xml" attachpoint="garrisoned"/>