qt4: fix y-axis positioning bug in the time tooltip

Reported by hivolbill in the forum.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/util/input_slider.cpp

@@ -298,7 +298,7 @@ void SeekSlider::mouseMoveEvent( QMouseEvent *event )
         }
 
         QPoint target( event->globalX() - ( event->x() - posX ),
-                  QWidget::mapToGlobal( pos() ).y() );
+                  QWidget::mapToGlobal( QPoint( 0, 0 ) ).y() );
         secstotimestr( psz_length, ( ( posX - margin ) * inputLength ) / ( size().width() - handleLength() ) );
         mTimeTooltip->setTip( target, psz_length, chapterLabel );
     }