shlibafsrpc: correct double export in symbol map

afs_xdr_pointer was exported twice. remove one.
1.6-only fix.

Change-Id: I55c6663340d0b87331f18b3d120d5184b567f5e1
Reviewed-on: http://gerrit.openafs.org/11551
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>