Get rid of QOpenGLContextPrivate::setGlobalShareContex

Manually import qt_(set_)gl_global_share_context from QtGui
instead.

The binary compatibility of those symbols should be maintained
in QtGui, allowing us to avoid depending on private API.

Change-Id: I1f954ec10a793c3195d3aec01409dde1bc767d64
Reviewed-by: Andras Becsi <andras.becsi@digia.com>