cmake: Don't install Window*h and Item_p.h

They are unneeded, and expose kdbindings

src/qtcommon/Platform_qt.h

@@ -13,10 +13,11 @@
 
 #include "kddockwidgets/docks_export.h"
 #include "kddockwidgets/Platform.h"
-#include "kddockwidgets/Window_qt.h"
 
 namespace KDDockWidgets {
 
+class Window;
+
 /// @brief Baseclass platform for Qt based platform
 /// Useful since QtWidgets and QtQuick share some similarities
 class DOCKS_EXPORT Platform_qt : public Platform