fflush(stdout) to print line by line if stdout is not a tty

To reproduce the issue:
	$ slstatus -s | tee

then it would print only when the stdout buffer is full, by block
of multiple lines.

fflush() makes sure the line is printed at every loop iteration

slstatus.c

@@ -98,6 +98,7 @@ main(int argc, char *argv[])
 
 		if (sflag) {
 			printf("%s\n", status);
+			fflush(stdout);
 		} else {
 			XStoreName(dpy, DefaultRootWindow(dpy), status);
 			XSync(dpy, False);