[ticket/13406] Add a space between the index name and columns list

Currently there's no space between the index name and columns list
when generating ADD INDEX sql query for MySQL DBMSes. This may cause errors
on earlier MySQL versions like 3.23.

PHPBB3-13406
Merge pull request #3205 from rxu/ticket/13406

[ticket/13406] Add a space between the index name and columns list