ssh: tidy include handling

Centralize optional krb5_config.h handling in ssh.mk. Do not add
headers (that are committed to the src tree) to SRCS as there is no
need.

Reviewed by: imp, jlduran, kevans (all earlier)
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34409

(cherry picked from commit 7f916236044d9a733de8b3c47b5dcbf71988cb03)