Fix exposure of privately used module

The QtOpenGLExtensions was exposed in the public library
interface of QtQuick3D causing it to be linked into user applications,
the effect of this is that applications built (in debug mode) with Qt from
our binary installer will fail to build, as the build system will attempt
to pull in the debug version of QtOpenGLExtensions, which is not shipped
with the binary installed version of Qt (that is a separate issue).

Fixes: QTBUG-83184
Change-Id: Ib5c192fdf0c0297ae06920b39712f565a0bc2770
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>