Windows: do not call cm_SearchCellByDNS if AFS_AFSDB_ENV is not defined

Windows: do not call cm_SearchCellByDNS if AFS_AFSDB_ENV is not defined

cm_SearchCellByDNS is only available when AFS_AFSDB_ENV is
defined.  Do not call it when it isn't.

LICENSE MIT

Change-Id: I203ab775513bd75ff6a547c557579c0bc095d555
Reviewed-on: http://gerrit.openafs.org/1086
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>