Fix dependency loop: do not mark non-generated source files as generated

Summary:

• Since sqlscanner.cpp isn't generated, don't claim it is.
• The ninja backend for CMake detects dependency loops and refuses to build.
• Same caveat as the other lines commented with TODO(GEN) applies.

Reviewers: staniek

Tags: KDb

Maniphest Tasks: T3743

Differential Revision: https://phabricator.kde.org/D7572

FIXED-IN:3.1.0