sysutils/signon-ui: update to the latest commit (to switch to webengine)

Update to the latest commit:
4368bb77 from 5 years ago

This allows us to use the maintained QtWebEngine instead of the
unmaintained QtWebKit.

The version has been set to 0.17-355, which is somewhat reasonable given

git rev-list 0.17+15.10.20150810-0ubuntu1.. --count

355

Still, it is a fake version :)