Ambiguity between std::log(double) and std::log(float)

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit b7255573)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/interface_widgets.cpp

@@ -428,7 +428,7 @@ void SpeedControlWidget::updateControls( float rate )
         return;
     }
 
-    double value = 17 * log( rate ) / log( 2 );
+    double value = 17 * log( rate ) / log( 2. );
     int sliderValue = (int) ( ( value > 0 ) ? value + .5 : value - .5 );
 
     if( sliderValue < speedSlider->minimum() )