fix building by MSVC8

src/mail/ngx_mail_ssl_module.c

diff --git a/src/mail/ngx_mail_ssl_module.c b/src/mail/ngx_mail_ssl_module.c
index dbefb14..4e2bda5 100644
--- a/src/mail/ngx_mail_ssl_module.c
+++ b/src/mail/ngx_mail_ssl_module.c
@@ -182,7 +182,7 @@
      */
 
     scf->enable = NGX_CONF_UNSET;
-    scf->starttls = NGX_CONF_UNSET;
+    scf->starttls = NGX_CONF_UNSET_UINT;
     scf->prefer_server_ciphers = NGX_CONF_UNSET;
     scf->builtin_session_cache = NGX_CONF_UNSET;
     scf->session_timeout = NGX_CONF_UNSET;