openbox: fix sporadic race on compile

| make: creating data/xsession/openbox-gnome-session
| make: creating data/xsession/openbox-session
| make: creating data/xsession/openbox-kde-session
| make: creating data/autostart/openbox-autostart
| make: creating doc/openbox.1
| make: creating data/autostart/autostart
| make: creating doc/openbox-session.1
| make: creating doc/openbox-gnome-session.1
| make: creating doc/openbox-kde-session.1
| make: creating doc/obxprop.1
| mkdir: cannot create directory './data/autostart': File exists
| Makefile:4329: recipe for target 'data/autostart/autostart' failed

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>