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

(cherry picked from commit 61639bb3fc5abe0bb7b096e643b51c30703ac432)
(cherry picked from commit 1e3dfe0c343c74d4c7162e25378b221e5a797917)