ssh: correct configure option name

The option is security-key-builtin not security-key-internal. There is
no change to the generated config.h because the option defaults off
anyway.

MFC after: 3 days
Fixes: 87152f340549 ("ssh: disble internal security key...")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f1421a8972a2c8babfacfd1270258492579e0208)