MINOR: listener: add flags field

Define a new field in listener structure named flags.

For the moment, no flag is defined. This will be notably useful to
differentiate QUIC listeners with the implementation of a QUIC conn
accept queue.
diff --git a/include/haproxy/listener-t.h b/include/haproxy/listener-t.h
index ebdeb7c..6fffc4a 100644
--- a/include/haproxy/listener-t.h
+++ b/include/haproxy/listener-t.h
@@ -201,6 +201,7 @@
 	short int nice;                 /* nice value to assign to the instantiated tasks */
 	int luid;			/* listener universally unique ID, used for SNMP */
 	int options;			/* socket options : LI_O_* */
+	int flags;                      /* LI_F_* flags */
 	__decl_thread(HA_SPINLOCK_T lock);
 
 	struct fe_counters *counters;	/* statistics counters */