libc: move NIS xdr_* symbols from rpc's to yp's Symbol.map

To fix WITHOUT_NIS build. Building yp_xdr.c is gated by MK_NIS.

PR: 279270
Reported by: peterj
Reported by: matteo
Reported by: Michael Dexter's Build Option Survey run
Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45347