Update build.sh openldap version to 2.4.57.

build.sh

 #!/bin/bash -e
 # Maintainer: sfeng@stanford.edu
 
-OPENLDAP_VERSION=openldap-2.4.50
+OPENLDAP_VERSION=openldap-2.4.57
 CYRUS_SASL_VERSION=cyrus-sasl-2.1.27
 OPENLDAP_TARBALL=ftp://ftp.openldap.org/pub/OpenLDAP/openldap-release/${OPENLDAP_VERSION}.tgz
 CYRUS_SASL_TARBALL=https://github.com/cyrusimap/cyrus-sasl/releases/download/${CYRUS_SASL_VERSION}/${CYRUS_SASL_VERSION}.tar.gz