Fix a strict warnings error in rand_pool_acquire_entropy

There was a warning about unused variables in this config:
./config --strict-warnings --with-rand-seed=rdcpu

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9687)