Fix missing shadows on C/C++ header mimetype icons

Summary:
Some shadow objects had no fill because the gradients they referenced were missing.

BUG: 401793
FIXED-IN: 5.56

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18837