owncloud: add some headers

Now there's no need to put these in your vhost. You'll get rid of warning messages.

owncloud/nginx.conf

@@ -14,6 +14,10 @@ http {
 
     access_log off;
     error_log /var/log/nginx/error.log error;
+    add_header Strict-Transport-Security "max-age=15552000; includeSubdomains; prel$
+    add_header X-Frame-Options SAMEORIGIN;
+    add_header X-Content-Type-Options nosniff;
+    add_header X-XSS-Protection "1; mode=block";
 
     sendfile on;
     keepalive_timeout 15;