Generate a moc_predefs.h file for KIOCore

Summary:
Otherwise moc gets through wrong files in some platforms that have weird
preprocessor hacks to support large files on 32 bits systems.
This should be done by automoc.

See:
https://gitlab.kitware.com/cmake/cmake/issues/16640
https://bugreports.qt.io/browse/QTBUG-57796

BUG: 371721

Test Plan: Fixes build of KIO on ARM32 (flatpak)

Reviewers: Frameworks, dfaure, cgilles

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5319