Call XSync in redraw

It is necessary call to XSync if you want a good tput flash, because in
other way you can not be sure that white screen will be shown.
---
 st.c |    1 +
 1 file changed, 1 insertion(+)

st.c

@@ -2150,6 +2150,7 @@ redraw(void) {
 	struct timespec tv = {0, REDRAW_TIMEOUT * 1000};
 	tfulldirt();
 	draw();
+	XSync(xw.dpy, False); /* necessary for a good tput flash */
 	nanosleep(&tv, NULL);
 }