[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.

PHPBB3-11697
Merge remote-tracking branch 'galaxyAbstractor/ticket/11697' into develop

* galaxyAbstractor/ticket/11697:
  [ticket/11697] author_search() used incorrect parameter