const-correct a few SSL_SESSION functions.

Change-Id: I64d1e29d1e414add4ae522fd9603c550d426a557
Reviewed-on: https://boringssl-review.googlesource.com/5881
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/ssl_session.c b/ssl/ssl_session.c
index d452618..f4d8b34 100644
--- a/ssl/ssl_session.c
+++ b/ssl/ssl_session.c
@@ -218,11 +218,13 @@
   return session->time;
 }
 
-uint32_t SSL_SESSION_get_key_exchange_info(SSL_SESSION *session) {
+uint32_t SSL_SESSION_get_key_exchange_info(const SSL_SESSION *session) {
   return session->key_exchange_info;
 }
 
-X509 *SSL_SESSION_get0_peer(SSL_SESSION *session) { return session->peer; }
+X509 *SSL_SESSION_get0_peer(const SSL_SESSION *session) {
+  return session->peer;
+}
 
 long SSL_SESSION_set_time(SSL_SESSION *session, long time) {
   if (session == NULL) {