ssh: Update config.h for OpenSSL ED25519

OpenSSH-portable had a configure bug that prevented it from detecting
OpenSSL ED25519 support, fixed in 8d0e46c1ddb5 ("Fix OpenSSL ED25519
support detection"). This will come in with the OpenSSH 9.8p1 update,
but fix the error in config.h now.

Reported by: jlduran
Reviewed by: jlduran
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48942

(cherry picked from commit 975c6f3337039d14ecf87d674af72ac5ab0fee02)