Add phpinfo.php to 3.2.x install directory

In 3.1.x there's a phpinfo.php in the /install folder which seems to be removed in 3.2.x, perhaps by accident?

https://github.com/phpbb/phpbb/commits/3.2.x/phpBB/install/phpinfo.php

Let's add it back for easier troubleshooting potential issues during installation.