RAND_POOL: Add missing implementations for djgpp

Calling the functions rand_pool_add_{additional,nonce}_data()
in crypto/rand/rand_lib.c with no implementation for djgpp/MSDOS
causees unresolved symbols when linking with djgpp.

Reported and fixed by Gisle Vanem

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6421)