Fix size of the header in GridViewKCM

Summary:
The same as in SimpleKCM and ScrollviewKCM

BUG: 417347
FIXED-IN: 5.68

Test Plan:

Reviewers: mart, broulik, ngraham

Reviewed By: ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27315