Add MBEDTLS_PRIVATE wrapping to sample programs.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
diff --git a/programs/x509/cert_app.c b/programs/x509/cert_app.c
index fb24843..2adef39 100644
--- a/programs/x509/cert_app.c
+++ b/programs/x509/cert_app.c
@@ -335,7 +335,7 @@
 
             mbedtls_printf( "%s\n", buf );
 
-            cur = cur->next;
+            cur = cur->MBEDTLS_PRIVATE(next);
         }
 
         /*