Database: Explicitly globalize `$wpdb` in `wp-settings.php` in case WordPress isn't loaded in global scope.

See [18993] for `$table_prefix`.

Props danielbachhuber.
Fixes #37123.

git-svn-id: https://develop.svn.wordpress.org/trunk@37864 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-settings.php

@@ -99,6 +99,7 @@ require( ABSPATH . WPINC . '/class-wp-error.php' );
 require( ABSPATH . WPINC . '/pomo/mo.php' );
 
 // Include the wpdb class and, if present, a db.php database drop-in.
+global $wpdb;
 require_wp_db();
 
 // Set the database table prefix and the format specifiers for database table columns.