qt: remove use of Qt's QOpenGLFunctions

We already have our own versions so use them instead so that we don't
have the potential for redefinition warnings between system GL headers
and Qt's internal headers.

Fixes gstreamer/gst-plugins-good#523
3 jobs from qt-build-gl in 6 minutes (queued for 3 minutes and 56 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #54062
manifest

00:00:25

 
  Build
passed #54063
build fedora x86_64

00:02:41

 
  Full Builds
passed #54064
build android arm64 api28

00:02:53