qt: Ensure GL headers are included

Otherwise there may be no valid typedef of GLsync.

...
/usr/include/gstreamer-1.0/gst/gl/gstglfuncs.h:93:24: note: in definition of macro 'GST_GL_EXT_FUNCTION'
   ret (GSTGLAPI *name) args;
                        ^~~~
/usr/include/gstreamer-1.0/gst/gl/glprototypes/sync.h:33:23: error: 'GLsync' has not been declared
                      (GLsync sync))
                       ^~~~~~
...

https://bugzilla.gnome.org/show_bug.cgi?id=796879
1 file changed