rand_unix.c: fix --with-rand-seed=none build

Fixes a compiler warning about an unused syscall_random()
and cleans up the OPENSSL_RAND_SEED preprocessor logic.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/779)

(cherry picked from commit d90e128b)