Fix build with -no-opengl configured Qt

Commit e2541ebd

 introduced an unguarded
reference to QOpenGLContext. Guard it with the appropriate flags.

Task-number: QTBUG-57374
Change-Id: Ic9f0cc3c99aabd52813d8b828f1a51ca6871d4b4
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>