build: Add phan stub for ReturnTypeWillChange

ReturnTypeWillChange is new in php8.1
Allow to run phan under php80 with a stub

Change-Id: Ib84b314bf495634583b625ed8d7530908286e839

.phan/config.php

@@ -28,6 +28,7 @@ $cfg['minimum_target_php_version'] = '7.4.3';
 $cfg['file_list'] = array_merge(
 	$cfg['file_list'],
 	class_exists( Socket::class ) ? [] : [ '.phan/stubs/Socket.php' ],
+	class_exists( ReturnTypeWillChange::class ) ? [] : [ '.phan/stubs/ReturnTypeWillChange.php' ],
 	class_exists( AllowDynamicProperties::class ) ? [] : [ '.phan/stubs/AllowDynamicProperties.php' ],
 	[
 		// This makes constants and globals known to Phan before processing all other files.

.phan/stubs/ReturnTypeWillChange.php

@@ -0,0 +1,6 @@
+<?php
+
+// Stub for PHP 8.1's ReturnTypeWillChange class
+#[Attribute]
+class ReturnTypeWillChange {
+}