Define constant in use

[oandregal]

Fixes https://core.trac.wordpress.org/ticket/53175#comment:34

Steps to reproduce:

• Use a WordPress environment with PHP 8.
• Activate the debug logs.
• Set error_reporting( -1 ); at the head of load-styles.php.
• Visit /wp-admin/load-styles.php?load=wp-components

The expected result is that the styles load and that there's no message in the log.

Previous to this change, the styles won't load and you'll see this error in the log Fatal error: Uncaught Error: Undefined constant "WP_CONTENT_DIR".

[SergeyBiryukov]

Thanks for the PR! Closing in favor of #1325, which seems like a better option :)