Const-correct ECDH_compute_key.

Change-Id: Id8099cc3a250e36e62b8a48e74706b75e5fa127c
Reviewed-on: https://boringssl-review.googlesource.com/11566
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/ecdh/ecdh.c

@@ -76,7 +76,7 @@
 
 
 int ECDH_compute_key(void *out, size_t outlen, const EC_POINT *pub_key,
-                     EC_KEY *priv_key,
+                     const EC_KEY *priv_key,
                      void *(*kdf)(const void *in, size_t inlen, void *out,
                                   size_t *outlen)) {
   const BIGNUM *const priv = EC_KEY_get0_private_key(priv_key);

include/openssl/ecdh.h

@@ -85,10 +85,9 @@ extern "C" {
  * return value. Otherwise, as many bytes of the shared key as will fit are
  * copied directly to, at most, |outlen| bytes at |out|. It returns the number
  * of bytes written to |out|, or -1 on error. */
-OPENSSL_EXPORT int ECDH_compute_key(void *out, size_t outlen,
-                                    const EC_POINT *pub_key, EC_KEY *priv_key,
-                                    void *(*kdf)(const void *in, size_t inlen,
-                                                 void *out, size_t *outlen));
+OPENSSL_EXPORT int ECDH_compute_key(
+    void *out, size_t outlen, const EC_POINT *pub_key, const EC_KEY *priv_key,
+    void *(*kdf)(const void *in, size_t inlen, void *out, size_t *outlen));
 
 
 #if defined(__cplusplus)