Remove remaining boringssl constant from client.h

Follow-up to https://roughtime-review.googlesource.com/c/roughtime/+/2200

Change-Id: I27b411d493e1b3f0b188d24e71fd2d103a046960

client.h

diff --git a/client.h b/client.h
index fbe22ac..b1dd3de 100644
--- a/client.h
+++ b/client.h
@@ -30,7 +30,7 @@
 // and a certificate signed with a private key that matches |root_public_key|.
 bool ParseResponse(uint64_t *out_time, uint32_t *out_radius,
                    std::string *out_error,
-                   const uint8_t root_public_key[ED25519_PUBLIC_KEY_LEN],
+                   const uint8_t root_public_key[kPublicKeyLength],
                    const uint8_t *response_bytes, size_t response_len,
                    const uint8_t nonce[kNonceLength]);