KConfigCompiler_Test: Update test_signal.h.ref to follow the generated file

Summary:
Since "Use override" (3985f2a0) kconfig_compiler.cpp
generates usrSave with override instead of Q_DECL_OVERRIDE. This causes
the KConfigCompiler_Test to fail [1], as the reference file of the test
was not updated.

[1]: https://build.kde.org/job/Frameworks%20kconfig%20kf5-qt5%20SUSEQt5.10/32/

Reviewers: Frameworks, mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12892