volinfo: Include nfs.h

nfs.h is required for various types used in vol-info.c. Include it. On
namei this header gets pulled in indirectly via other means, but on
inode it does not (and we shouldn't be relying on such anyway).

Reviewed-on: http://gerrit.openafs.org/5185
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 2cd9d0536b173bf8c983aecd8599de0a82610151)

Change-Id: I8c4b1c096b617a3ed62b8e1c7b8677ee7de2a56e
Reviewed-on: http://gerrit.openafs.org/11262
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>