Fix the OverviewMap control & example

Layers must now always be specified for the control.
Base example was amended to reflect that.

The breaking change was added in the update notes.

examples/overviewmap.js

@@ -4,13 +4,22 @@ import {defaults as defaultControls, OverviewMap} from '../src/ol/control.js';
 import TileLayer from '../src/ol/layer/Tile.js';
 import OSM from '../src/ol/source/OSM.js';
 
+const source = new OSM();
+const overviewMapControl = new OverviewMap({
+  layers: [
+    new TileLayer({
+      source: source
+    })
+  ]
+});
+
 const map = new Map({
   controls: defaultControls().extend([
-    new OverviewMap()
+    overviewMapControl
   ]),
   layers: [
     new TileLayer({
-      source: new OSM()
+      source: source
     })
   ],
   target: 'map',