Added DT_EXPLOSION to warn list

DT_EXPLOSION should not be used in common code and
the usage checked by reviewers.
diff --git a/warn_prop.list b/warn_prop.list
index f0ed2ec..d95216f 100644
--- a/warn_prop.list
+++ b/warn_prop.list
@@ -5,3 +5,4 @@
 P_SKILL_ATTRIBUTES
 P_MAX_HANDS
 P_EXITS
+DT_EXPLOSION