another small optimisation

dwm.c

@@ -1236,7 +1236,7 @@ propertynotify(XEvent *e) {
 		if(ev->atom == XA_WM_NAME || ev->atom == netatom[NetWMName]) {
 			updatetitle(c);
 			if(c == c->mon->sel)
-				drawbars();
+				drawbar(c->mon);
 		}
 	}
 }