DOXYGEN_PREPROC -> DOXYGEN_SHOULD_SKIP_THIS, to standardize within KF5

Summary:
The other KF5 modules use DOXYGEN_SHOULD_SKIP_THIS, picking up the example
from the doxygen documentation.
So for consistency and current expectations of kapidox (and built-in support
of ECMAddQCH) switch to use that preprocessor macro as well.

Test Plan: Library still builds and works, generated QCH file still same content.

Reviewers: Frameworks, ilic

Reviewed By: ilic

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6338