liboafs_ubik: Export DISK_GetFile

The symbols StartDISK_GetFile and EndDISK_GetFile can be useful to
ubik users. Put them in the export list.

Change-Id: I6aa9c7261e68d66de19981154a7c42efc69e91f2
Reviewed-on: http://gerrit.openafs.org/9725
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>