Add missing type annotation in ol.format.WMSCapabilities

The error reported by the compiler was:
```
ERR! compile src/ol/format/wmscapabilitiesformat.js:160: ERROR - inconsistent return type
ERR! compile found   : Array
ERR! compile required: (null|ol.Extent|undefined)
ERR! compile   return [
ERR! compile          ^
ERR! compile
```

src/ol/format/wmscapabilitiesformat.js

@@ -157,10 +157,10 @@ ol.format.WMSCapabilities.readEXGeographicBoundingBox_ =
       !goog.isDef(eastBoundLongitude) || !goog.isDef(northBoundLatitude)) {
     return undefined;
   }
-  return [
+  return /** @type {ol.Extent} */ ([
     westBoundLongitude, southBoundLatitude,
     eastBoundLongitude, northBoundLatitude
-  ];
+  ]);
 };