Coding Standards: Use __DIR__ instead of ABSPATH in wp-db.php.

If the file is included directly outside of WordPress core, `ABSPATH` may not be defined.

Follow-up to [53749], [53750], [53755].

See #56268, #55647.

git-svn-id: https://develop.svn.wordpress.org/trunk@53756 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -14,4 +14,4 @@ if ( function_exists( '_deprecated_file' ) ) {
 }
 
 /** wpdb class */
-require_once ABSPATH . 'wp-includes/class-wpdb.php';
+require_once __DIR__ . '/class-wpdb.php';