Fix build with ICC 17: problem parsing override keyword

So just use the typedef that QPlatformOpenGLContext defines for a
function pointer.

qwaylandglcontext.h(76): error: expected a ";"

Intel Issue ID: 6000161944
Change-Id: I149e0540c00745fe8119fffd1463ce5c4ffdeb4c
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Johan Helsing <johan.helsing@qt.io>
Reviewed-by: Giulio Camuffo <giulio.camuffo@kdab.com>