vol: remove warning on use of DFlushVolume

vol: remove warning on use of DFlushVolume

Include the afs/dir.h header on Windows to avoid
warnings caused by the missing prototype.

Reviewed-on: http://gerrit.openafs.org/4148
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit d69da816c6e961ef5e22df289ad5667cd8568b4c)

Change-Id: I1f128f676a6d099b2d9534e28c6302de78b0ff2d
Reviewed-on: http://gerrit.openafs.org/6273
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>