xwiishow: handle EINTR from poll()

We get SIGWINCH whenever the master tty resizes the terminal. If this is
catched while poll() sleeps we get EINTR. Hence, catch this instead of
terminating the application.

Note that ncurses handles the signal automatically and so the keyboard()
helper will notify us about the resize event.

Reported-by: Andreas Fuchs <anduchs@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>