Add DesktopEntry to notifyrc

Summary:
This allows the notification KCM to identify akregator as an
application.

Test Plan:
Checked that akregator shows up under applications in the
new KCM.

Reviewers: #kde_pim, broulik

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21117