Test including rpc/xdr.h requires sunrpc

...which is set based on HAVE_RPC_XDR_H.  At least Fedora 32 does not have a
/usr/include/rpc/xdr.h, so failed this test introduced with
<https://reviews.llvm.org/D83358> "[Sanitizers] Add interceptor for
xdrrec_create".

Differential Revision: https://reviews.llvm.org/D84740