Const-correct HKDF_expand.

prk should be a const parameter.

Change-Id: I2369ed9f87fc3c59afc07d3b667b86aec340052e
Reviewed-on: https://boringssl-review.googlesource.com/8810
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/include/openssl/hkdf.h b/include/openssl/hkdf.h
index a484a30..bffb01e 100644
--- a/include/openssl/hkdf.h
+++ b/include/openssl/hkdf.h
@@ -50,7 +50,7 @@
  * |out_len| from the PRK |prk| and info |info| using |digest|, and outputs
  * the result to |out_key|. It returns one on success and zero on error. */
 OPENSSL_EXPORT int HKDF_expand(uint8_t *out_key, size_t out_len,
-                               const EVP_MD *digest, uint8_t *prk,
+                               const EVP_MD *digest, const uint8_t *prk,
                                size_t prk_len, const uint8_t *info,
                                size_t info_len);