According to QUIP-18 [1], all build system
files should be BSD-3-Clause

[1]: https://contribute.qt-project.org/quips/18



Pick-to: 6.7
Task-number: QTBUG-121787
Change-Id: I2f13ceaa8eb68b7745cffc66651f892c1fbeeff2
Reviewed-by: Kai Köhne <kai.koehne@qt.io>