make ecp_nistz256_point_add_vis3() local

fixes #8936

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9132)

(cherry picked from commit 861335001b99cfd665030c50ad37a663536a5c0f)