Fix signatures of EVP_Digest{Sign,Verify}Update

These are implemented as macros delegating to `EVP_DigestUpdate`, which
takes a `size_t` as its third argument, not an `unsigned int`.

CLA: trivial

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 8bdce8d1)