Added P_EXITS to list of warn properties

Accessing this property should be checked for necessity.
diff --git a/warn_prop.list b/warn_prop.list
index f8142c4..f0ed2ec 100644
--- a/warn_prop.list
+++ b/warn_prop.list
@@ -4,3 +4,4 @@
 P_PREVENT_PILE
 P_SKILL_ATTRIBUTES
 P_MAX_HANDS
+P_EXITS