Initialize static variable to make linter happy

Summary: Uninitialized variable defaults to 0 in this context. Make it explicit.

Test Plan: None. No functional change.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25470

src/applications/policy/filter/PhabricatorPolicyFilter.php

@@ -461,7 +461,7 @@ final class PhabricatorPolicyFilter extends Phobject {
     // checks make it difficult to create cycles normally, so just do a
     // simple check here to limit damage.
 
-    static $depth;
+    static $depth = 0;
 
     $depth++;