Fix Kyber segfaulting on Win64

* cipher/kyber.c (prg, pkprf): Cast variadic parameters to expected
types ('void *' and 'size_t').

Mismatch of variadic parameter integer types (int vs size_t) caused
shake256v to segfault on Win64.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>