Strict standards: Non-static method quick_edit_grade::filter()

Without this patch I get this PHP warning:

Strict standards: Non-static method quick_edit_grade::filter() should not be called statically in /vagrant/moodle/grade/report/quick_edit/lib.php on line 69

The "filter" method is defined as being non-static, but is being called statically. So I just changed the definition to also be static.