[php] Add extern for `filesize()` (#9832)

std/php/Global.hx

@@ -705,6 +705,11 @@ extern class Global {
 	**/
 	static function filemtime(filename:String):EitherType<Int, Bool>;
 
+	/**
+		@see http://php.net/manual/en/function.filesize.php
+	**/
+	static function filesize(filename:String):EitherType<Int, Bool>;
+
 	/**
 		@see http://php.net/manual/en/function.clearstatcache.php
 	**/