Add X-KDE-DBUS-Restricted-Interfaces to desktop file

After https://phabricator.kde.org/D29487 and in preparation of https://phabricator.kde.org/D29407

Will allow spectacle to take screenshots without confirmation click D29407 has landed.

Extracted from https://phabricator.kde.org/D29408

@davidre @ngraham @apol

This can land already after kservice patch frameworks/kservice@cdad4f73