qt5quickcontrols: fix LTS (5.6) qml file install

Fixes #9666 [1]

  cp -dpfr .../host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/qml/QtQuick/PrivateWidgets .../target/usr/qml/QtQuick
  cp: cannot stat ‘.../host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/qml/QtQuick/PrivateWidgets’: No such file or directory

[1] https://bugs.busybox.net/show_bug.cgi?id=9666



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>