allow qan-app to be opened in iframe

Please augment QAN location in nginx conf with header 'X-Frame-Options "SAMEORIGIN"'

Ex.:

/etc/nginx/conf.d/pmm.conf

1. QAN App
   location /qan {
   add_header X-Frame-Options "SAMEORIGIN";
   alias /usr/share/percona-qan-app;
   try_files $uri$args $uri$args/ /index.html break;
   }