Fix incorrect version in @since tag

Summary:
The following functions only appeared in v5.39, however their @since
tag specified v5.38.

KIconLoader::setCustomPalette()
KIconLoader::customPalette()
KIconLoader::resetPalette()

Test Plan: Ensure the above functions didn't exist in v5.38

Reviewers: Frameworks, cfeck

Reviewed By: cfeck

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12650