Fix ico.json to avoid returning an empty mimetype

QImageWriter::supportedMimeTypes() had an empty mimetype name
in the list because the mimetype for the key "cur" was missing in this
json file.

Change-Id: I4eae4275cb04c4d640dbcac76cd9dc99baa4f0a7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/imageformats/ico/ico.json

@@ -1,4 +1,4 @@
 {
     "Keys": [ "ico", "cur" ],
-    "MimeTypes": [ "image/vnd.microsoft.icon" ]
+    "MimeTypes": [ "image/vnd.microsoft.icon", "image/vnd.microsoft.icon" ]
 }