Export xdr_nbulkentries from liboafs_vldb

Change-Id: I117b875189f4f9de971a99ff68eca470515a11ad
Reviewed-on: http://gerrit.openafs.org/11449
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>