fix: do not use Implicit QML function call

Qt 5.15 introduced new syntax for defining Connections. Fix warnings like this one:
    QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }

follow up of d2f3372749

scripts/desktopchangeosd/contents/ui/osd.qml

@@ -276,7 +276,9 @@ PlasmaCore.Dialog {
 
         Connections {
             target: workspace
-            onCurrentDesktopChanged: dialogItem.show()
+            function onCurrentDesktopChanged() {
+                dialogItem.show()
+            }
             function onNumberDesktopsChanged() {
                 repeater.model = workspace.desktops;
             }