libafsrpc: export more xdr functions

libafsrpc: export more xdr functions

Most of the xdr functions in the library text are to support RXAFS and
RXAFSCB RPCs, which we explicitly do not expose from libafsrpc.
As such, they do not need to be in the export list, but a couple of
generic ones probably should be exported.

Do so, for both Unix and Windows.

Change-Id: I12ddf2427d807f4ee7b07af1e1c498fc119a0f1c
Reviewed-on: https://gerrit.openafs.org/13139
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>