[rspamd] Restore add header forced action (#3440)

Revert 0474de88b175ef1cb638ef5525376033057be564. Fixed since c3a4c6d311058103287a4ad95ab2cca0bc51585a.

data/conf/rspamd/local.d/force_actions.conf

@@ -1,6 +1,6 @@
 rules {
   WHITELIST_FORWARDING_HOST_NO_REJECT {
-    action = "rewrite subject";
+    action = "add header";
     expression = "WHITELISTED_FWD_HOST";
     require_action = ["reject"];
   }