Set screenshot URL for notification also in stable branch

We made a huge fuzz about this feature in the 5.9 release announcement
yet Spectacle that supports this isn't to be released until April.

Since it's just a one-liner let's add this feature to stable branch with an
ifdef for KNotifications version (it was introduced in 5.29)

Differential Revision: https://phabricator.kde.org/D4390