rpc: Fix the definition of xdr_void()

xdr_void() should have type xdrproc_t, make it so.

Approved by: re (kib)
PR: 280514
Reviewed by: brooks, dim
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47340

(cherry picked from commit a5d1cf5e362a2e3c3ebdf6d8f2b86658a6d0b9d6)
(cherry picked from commit b698c825f3b6699b278122da776bdfe5f360619f)