MINOR: stream-int: implement the stream_int_notify() function

stream_int_notify() was taken from the common part between si_conn_wake_cb()
and si_applet_done(). It is designed to report activity to a stream from
outside its handler. It'll generally be used by lower layers to report I/O
completion but may also be used by remote streams if the buffer processing
is shared.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index d1d75ba..22d05be 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -51,6 +51,7 @@
 void stream_int_update(struct stream_interface *si);
 void stream_int_update_conn(struct stream_interface *si);
 void stream_int_update_applet(struct stream_interface *si);
+void stream_int_notify(struct stream_interface *si);
 
 /* returns the channel which receives data from this stream interface (input channel) */
 static inline struct channel *si_ic(struct stream_interface *si)