Add missing prototype for FIPS callback

Fixes #2533

The call to FIPS_crypto_set_id_callback() was added in revision a43cfd7b,
but there is no prototype for it in <openssl/fips.h>.

Signed-off-by: Dr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4870)