Add 'Prophecy' namespace to the scoping whitelist

In oder to setup or check test doubles with Prophecy it has to be
part of the public API of PHPUnit, even it not provided by PHPUnit
itself.

Issues:
- #3578 - Scoped PHAR Prophecy issue

build/scoper.inc.php

@@ -13,6 +13,7 @@
     'SebastianBergmann\CodeCoverage\*',
     'PharIo\*',
     'PHP_Token*',
+    'Prophecy\*',
 ];
 
 return [