Add SystemPalette to ProgressBar

The progress bar had no system palette but referred to one.

components/ProgressBar.qml

@@ -12,6 +12,8 @@ Components.ProgressBar {
     height: orientation === Qt.Horizontal ? sizehint.height : 200
     width: orientation === Qt.Horizontal ? 200 : sizehint.height
 
+    SystemPalette {id: syspal}
+
     background: QStyleItem {
         anchors.fill: parent
         elementType: "progressbar"