Fix SPDX license identifier
'LicenseRef-KDE-Accepted-GPL' alone is not valid. The correct syntax is 'GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL'
See: https://community.kde.org/Policies/Licensing_Policy#GPL-3.0-only_OR_LicenseRef-KDE-Accepted-GPL
'LicenseRef-KDE-Accepted-GPL' alone is not valid. The correct syntax is 'GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL'
See: https://community.kde.org/Policies/Licensing_Policy#GPL-3.0-only_OR_LicenseRef-KDE-Accepted-GPL