Declare missing class properties

Summary: Code in these classes tries to access undefined properties. Thus define these properties.

Test Plan: Run PHPStan static code analysis; grep the code in each class.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25712

src/future/postmark/PhutilPostmarkFuture.php

@@ -3,6 +3,7 @@
 final class PhutilPostmarkFuture extends FutureProxy {
 
   private $future;
+  private $clientID;
   private $accessToken;
   private $method;
   private $parameters;

src/workingcopy/ArcanistWorkingCopyPath.php

@@ -7,6 +7,7 @@ final class ArcanistWorkingCopyPath
   private $mode;
   private $data;
   private $binary;
+  private $mimeType;
   private $dataAsLines;
   private $charMap;
   private $lineMap;