gssd: Add support for MIT KRB5

Fix gssd build when building with MIT KRB5.

krb5_get_init_creds_opt_set_default_flags() is not an MIT KRB5 function.
It is also not needed to resolve the default keytable.

Sponsored by: The FreeBSD Foundation
Reviewed by: rmacklem
Differential revision: https://reviews.freebsd.org/D50810