...to avoid a false positive in --enable-qt5

> [build CXX] vcl/qt5/Qt5Widget.cxx
> /home/sbergman/lo/core/vcl/qt5/Qt5Widget.cxx:466:21: error: Unreferenced function declaration [loplugin:unreffun]
>     friend QWidget* createQt5Widget(Qt5Frame& rFrame, QWidget* parent, Qt::WindowFlags f);
>     ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 1 error generated.

Change-Id: I01e47bc5f5147916737d62fe18ce5312a0221a3f
Reviewed-on: https://gerrit.libreoffice.org/55189Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>