Use correct include guard for vol_prototypes.h

Correct the case of the 'h'.

Change-Id: Ib78cbff02ac881aa54c46832db94094f7a74ff5f
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>