ADD INDEX syntax may cause an error in earlier MySQL versions

Currently there's no space between the index name and columns list when generating ADD INDEX sql query for MySQL DBMSes
https://github.com/phpbb/phpbb/blob/develop/phpBB/phpbb/db/tools.php#L2178

$statements[] = 'ALTER TABLE ' . $table_name . ' ADD INDEX ' . $index_name . '(' . implode(', ', $column) . ')';

This may cause errors on earlier MySQL versions like 3.23 (which is supported by phpBB 3.1) like

SQL ERROR [ mysql ]

You have an error in your SQL syntax near 'user(user_id, notification_read)' at line 1 [1064]

SQL

ALTER TABLE phpbb3_notifications ADD INDEX user(user_id, notification_read)

Adding a space solves the issue.