wayland-egl: reuse the existing WL_EXPORT macro

There's little point if redefining new one ourselves. Just reuse the one
that's already available.

Cc: Daniel Stone <daniels@collabora.com>
Suggested-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Arnaud Vrac <avrac@freebox.fr>