On OpenBSD, use arc4random_buf() instead of rand() to prevent warnings

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 5b1529c..2f5be94 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -24,6 +24,7 @@
    * Improve interoperability by not writing extension length in ClientHello /
      ServerHello when no extensions are present (found by Matthew Page)
    * rsa_check_pubkey() now allows an E up to N
+   * On OpenBSD, use arc4random_buf() instead of rand() to prevent warnings
 
 = PolarSSL 1.3.6 released on 2014-04-11