KService: fix kded compilation with -DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x053f00

Summary:
kded needs to keep being able to load kded modules that install desktop
files rather than providing json metadata. Not all kded modules have
been ported away from that, see e.g. D13360 or print-manager.

NO_CHANGELOG

Test Plan: kservice builds

Reviewers: kossebau, vkrause, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24974