crypto/hash: export Hasher

Forgot to do this originally. Oops!

Bug: None
Change-Id: If799d64b8de834a27bd066934346c9e182381e83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6290228
Reviewed-by: Michael Cui <mlcui@google.com>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422932}

crypto/hash.h

@@ -55,7 +55,7 @@ CRYPTO_EXPORT void Hash(HashKind kind,
 
 // A streaming hasher interface. Calling Finish() resets the hash context to the
 // initial state after computing the digest.
-class Hasher {
+class CRYPTO_EXPORT Hasher {
  public:
   explicit Hasher(HashKind kind);
   Hasher(const Hasher& other);