MINOR: mux-h1: Add a trace on shutdown when keep-alive is not possible

When the stream is shut down, some tests are performed to know if the
connection must also be closed or not. There are trace messages for all
cases, except for the default one: Abort or close-mode. Thanks to this
patch, there is now a message too in this case.

src/mux_h1.c

@@ -1066,6 +1066,7 @@ static int h1s_must_shut_conn(struct h1s *h1s)
 	}
 	else {
 		/* The default case, do the shutdown */
+		TRACE_STATE("shutdown on connection (abort || want_clo)", H1_EV_STRM_SHUT, h1c->conn, h1s);
 		ret = 1;
 	}