Declare undeclared variable in lint engine

Test Plan:
  $ arc lint

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3876

src/lint/engine/ArcanistLintEngine.php

@@ -61,6 +61,7 @@
 abstract class ArcanistLintEngine {
 
   protected $workingCopy;
+  protected $paths = array();
   protected $fileData = array();
 
   protected $charToLine = array();