Incorrect string concatenation in phpbb_mcp_sorting()

If a user does not have m_approve permissions in the MCP, he/she will only see the approved topics. For that, we specify the topic_visibility for the query. However, due to incorrect string concatenation there is no space before the previous topic_last_post_time value and the AND:
https://github.com/phpbb/phpbb/blame/3.1.x/phpBB/includes/functions_mcp.php#L391