Fix -Wcast-align in crypto_hash.cpp

Summary:
Avoid the C-style cast to get rid of the warning.

This does not change the behavior.

Test Plan:

ninja bitcoin-bench

Check the warning disappeared.

./src/bench/bitcoin-bench -filter=SipHash_32b

Compare the benchmark result with master and check there is no
performance penalty.

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4680