[ticket/11697] author_search() used incorrect parameter

The author_search() function in fulltext_mysql.php had an argument
$m_approve_fid_ary parameter but it was changed in the Docblock
and code to $post_visibility. This change renames that argument
according to the docblock, the code and to mirror the other classes.

http://tracker.phpbb.com/browse/PHPBB3-11697

PHPBB3-11697