Fix warning when building with Qt 4

Change-Id: I52b6b84732cf363b06ea248eeabdc99d89fb4beb
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/libs/utils/stylehelper.cpp

@@ -369,6 +369,7 @@ static qreal pixmapDevicePixelRatio(const QPixmap &pixmap)
 #if QT_VERSION > 0x050000
     return pixmap.devicePixelRatio();
 #else
+    Q_UNUSED(pixmap);
     return 1.0;
 #endif
 }