[nightcolor] Add missing const qualifiers

Summary: Mark some immutable variables with const to improve code readability.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson, romangg

Reviewed By: #kwin, davidedmundson, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24235