Fix compilation

Summary:
Seems only gcc can do a constexpr with strlen.

This fixes the build with clang, hopefully to with MSVC?

Reviewers: svuorela

Reviewed By: svuorela

Subscribers: svuorela, apol, pino, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20149