MINOR: stream_interface: add a pointer to the listener for TARG_TYPE_CLIENT

When the target is a client, it will be convenient to have a pointer to the
original listener so that we can retrieve some configuration information at
the stream interface level.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 8e9e64b..8218eda 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -122,6 +122,7 @@
 		struct server *s;     /* when type is TARG_TYPE_SERVER */
 		struct si_applet *a;  /* when type is TARG_TYPE_APPLET */
 		struct task *t;       /* when type is TARG_TYPE_TASK */
+		struct listener *l;   /* when type is TARG_TYPE_CLIENT */
 	} ptr;
 };