Given that the validation link is more of a dev thing than something customers need, and if issues need fixing then FTP will be required, there is no admin switch.

Instead, there are two file-based ways to enable it:

You could simply change the 'false' to 'true' in the file itself:

Code:
<?php if (false || (isset($showValidatorLink) && $showValidatorLink == true)) { ?>
Or, alternatively you could add an /includes/extra_configures/validator_link.php file, containing:
Code:
<?php
$showValidatorLink = true;
and simply change its extension from .php to .off when not needed. Or just leave it as .php if you're begging for a way to leave the icon on all the time.
(You could have this file on your "dev" site, and skip uploading it to your "live" site. That way you always have the "dev" site showing the icon for convenience.)