[kstyle] Require Qt 5.15

QWindow::startSystemMove, which was introduced with Qt 5.15, was added
with commit 6d83b045.

CMakeLists.txt

@@ -4,6 +4,7 @@ set(PROJECT_VERSION "5.19.90")
 set(PROJECT_VERSION_MAJOR 5)
 
 set(KF5_MIN_VERSION "5.74")
+set(QT_MIN_VERSION "5.15.0")
 
 include(GenerateExportHeader)
 include(WriteBasicConfigVersionFile)

kstyle/CMakeLists.txt

 ################# Qt/KDE #################
-find_package(Qt5 REQUIRED CONFIG COMPONENTS Widgets DBus)
+find_package(Qt5 ${QT_MIN_VERSION} REQUIRED CONFIG COMPONENTS Widgets DBus)
 find_package(KF5 REQUIRED COMPONENTS
     I18n
     Config