Include fcitx's platform input context in runtime.

Summary:
The latest fcitx platform input context added support for flatpak portal.
Include this in the runtime will enable user to use host fcitx with flatpak
Qt applications.

Test Plan: Manually test a local compiled runtime with org.kde.ark from kdeapss repo.

Reviewers: apol, hein

Reviewed By: apol

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D8739