wayland-drm: constify the callbacks struct, take 2

Now that wayland-drm (correctly) keeps a local copy of the callbacks,
this should not longer cause explosions.

After all the symbol is a local, constant data.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Tested-by: Derek Foreman <derekf@osg.samsung.com>
3 files changed