Revert "Remove duplicate xdr_vector() from afsaux.c"

This reverts commit 165f86ad529cf4f5de7029b741434cc90acaadca.
Remove duplicate xdr_vector from xdr_update.c

This is already present in afsaux.c

Change-Id: I8e469c9974e8786f8fa5a669cd7fe7f859bdd224