Use correct include guard for vol_prototypes.h

Correct the case of the 'h'.

Reviewed-on: http://gerrit.openafs.org/10889
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 8832cd4e61f04fa5c650060e968bd7834afebd92)

Change-Id: I306107184667724e48d5c9c32a11ddc4c9748ce7
Reviewed-on: http://gerrit.openafs.org/10924
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>