Remove preauth_sam2 from windows build

The source file preauth_sam2.c is not present on the krb5-1.10 branch,
so don't try building it.

Signed-off-by: Kevin Wasserman <kevin.wasserman@painless-security.com>

[tlyu@mit.edu: clarify commit message]

https://github.com/krb5/krb5/commit/b4e3cc07be2307f0561c9a95251e7c48c491d459
Author: Kevin Wasserman <kevin.wasserman@painless-security.com>
Committer: Tom Yu <tlyu@mit.edu>
Commit: b4e3cc07be2307f0561c9a95251e7c48c491d459
Branch: krb5-1.10
 src/lib/krb5/krb/Makefile.in |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)