D28056: KWindowSystem: Convert license headers to SPDX expressions

[Andreas Cord-Landwehr]

cordlandwehr added a task: T11550: Add SPDX License markers.

REPOSITORY
  R278 KWindowSystem

REVISION DETAIL
  https://phabricator.kde.org/D28056

To: cordlandwehr
Cc: kde-frameworks-devel, LeGast00n, cblack, GB_2, michaelh, ngraham, bruns