Add "random_int" and "random_bytes" as builtins so the linter stops complaining about them

Summary: There's a "right" way to do this by regenerating this file from PHPCompatInfo, but that requires installing composer and a whole lot of legwork. For now, I think we only actually care about these two functions, so I just added them manually.

Test Plan: On PHP7, ran arc lint on Filesystem.php and got no warnings. This change should also stop us from getting warnings on earlier versions of PHP.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20202