Fix a TODO(3.0) in the siphash code

Fix a TODO(3.0) in the siphash code

All 3 files that included crypto/siphash.h also included siphash_local.h,
and no other files included siphash_local.h independently. They probably
should be just one header file.

Fixes #14360

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14558)