MINOR: action: define enum for timeout type of the set-timeout rule

This enum is used to specify the timeout targetted by a set-timeout
rule.
diff --git a/include/haproxy/action-t.h b/include/haproxy/action-t.h
index 36aa5bb..73a846f 100644
--- a/include/haproxy/action-t.h
+++ b/include/haproxy/action-t.h
@@ -93,6 +93,12 @@
 	ACT_TCP_CLOSE, /* close at the sender's */
 };
 
+/* Timeout name valid for a set-timeout rule */
+enum act_timeout_name {
+	ACT_TIMEOUT_SERVER,
+	ACT_TIMEOUT_TUNNEL,
+};
+
 /* NOTE: if <.action_ptr> is defined, the referenced function will always be
  *       called regardless the action type. */
 struct act_rule {