const-correct a variable.

Change-Id: I00507014c55b2c7fd442a5aa2c3afcbf8c48049b
Reviewed-on: https://boringssl-review.googlesource.com/10741
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/ssl/tls13_enc.c b/ssl/tls13_enc.c
index 518d2d3..88fe8f0 100644
--- a/ssl/tls13_enc.c
+++ b/ssl/tls13_enc.c
@@ -318,7 +318,7 @@
   uint8_t key[EVP_MAX_MD_SIZE];
   size_t key_len = EVP_MD_size(digest);
 
-  uint8_t *traffic_secret;
+  const uint8_t *traffic_secret;
   const char *label;
   if (is_server) {
     label = "server finished";