libutil: Fix login_setcryptfmt()

Our dummy crypt_set_format() needs to be exported, otherwise it will
be resolved at link time instead of load time, preventing the real
crypt_set_format() in libcrypt from replacing it when present.

PR: 288677
Fixes: 0c381b7f0570
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D51746