vol: avoid defining unused struct on windows

vol: avoid defining unused struct on windows

namei_ogm_t is not used on Windows.  There is no equivalent
to owner/group mode.

Reviewed-on: http://gerrit.openafs.org/3698
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 48508a202129be6e56beaec56314cc13d68d69f5)

Change-Id: I6f7c5dc22e45f4da45ebc391512e4a296fe4b2ca
Reviewed-on: http://gerrit.openafs.org/3842
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>