ScreenSaver: Set target for turn off display checkbox

I noticed while testing the nightly on vmware which, unlike vbox, includes
a video driver with DPMS support that this checkbox wasn't working.
This should fix it.

src/preferences/screensaver/ScreenSaverWindow.cpp

@@ -379,6 +379,7 @@ FadeView::AttachedToWindow()
 {
 	fEnableCheckBox->SetTarget(this);
 	fRunSlider->SetTarget(this);
+	fTurnOffCheckBox->SetTarget(this);
 	fTurnOffSlider->SetTarget(this);
 	fFadeNow->SetTarget(this);
 	fFadeNever->SetTarget(this);