SSL: fixed build with OpenSSL 0.9.7.

SSL_get_rbio() and SSL_get_wbio() functions used to get non-const pointer
in OpenSSL 0.9.7, hence an explicit cast added to drop const qualifier.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index cd8f0e7..df8f7e1 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -548,8 +548,8 @@
              * added to wbio, and set buffer size.
              */
 
-            rbio = SSL_get_rbio(ssl_conn);
-            wbio = SSL_get_wbio(ssl_conn);
+            rbio = SSL_get_rbio((ngx_ssl_conn_t *) ssl_conn);
+            wbio = SSL_get_wbio((ngx_ssl_conn_t *) ssl_conn);
 
             if (rbio != wbio) {
                 (void) BIO_set_write_buffer_size(wbio, NGX_SSL_BUFSIZE);