Fix detection of GLX support in QtGui

I was using the QT_ENABLED_PUBLIC_FEATURES CMake variable wrongly,
QT_ENABLED_PUBLIC_FEATURES is a property defined for Qt targets.

Reported by Fushan Wen in
https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/419

Also move the add_feature_info(GLX) call to the same if block where we set
HAVE_GLX 1 (I was fooled by the build log saying "This feature is
enabled"... :|).