wayland-egl: rename wl_egl_window::private to driver_private

private is a reserved keyword in C++. Thus if one is to have a backend
driver written in said language build will fail as below:

.../wayland-egl-backend.h:56:8: expected unqualified-id before ‘private’

Rename it to driver_private and update the test.

NOTE: version bump is not required since:
 - this is a pure API change, ABI is identical
 - hardware drivers already require [minor] changes to move to the
upstream wayland-egl-backend.h

Cc: Arnaud Vrac <rawoul@gmail.com>
Cc: Miguel A . Vico <mvicomoya@nvidia.com>
Suggested-by: Arnaud Vrac <rawoul@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Miguel A Vico Moya <mvicomoya@nvidia.com>