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/crypto/hkdf/hkdf.c b/crypto/hkdf/hkdf.c
index d80834d..f21cb42 100644
--- a/crypto/hkdf/hkdf.c
+++ b/crypto/hkdf/hkdf.c
@@ -55,7 +55,7 @@
 }
 
 int HKDF_expand(uint8_t *out_key, size_t out_len, const EVP_MD *digest,
-                uint8_t *prk, size_t prk_len, const uint8_t *info,
+                const uint8_t *prk, size_t prk_len, const uint8_t *info,
                 size_t info_len) {
   /* https://tools.ietf.org/html/rfc5869#section-2.3 */
   const size_t digest_len = EVP_MD_size(digest);