Export xdr_nbulkentries in liboafs_vldb

Change-Id: Id6ba0e4fdb802cc10aa98b32d7e7c9b605c90606
Reviewed-on: http://gerrit.openafs.org/11334
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>