util/add-depends.pl: Only add dependencies on existing or generated headers
util/add-depends.pl: Only add dependencies on existing or generated headers

Headers that fulfill neither of those conditions are skipped.

This avoids build breaks when development has removed a previously
existing header.

Fixes #16360

Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16361)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl