Require the "ctype" extension explicitly

Summary: Fixes T4916. Although every normal build of PHP has this in the core, at least one distribution which users could reasonably encounter does not.

Test Plan: Changed string to "ctypex", got setup warning. Changed to "ctype", got no warning.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4916

Differential Revision: https://secure.phabricator.com/D8896