DirAccessWindows: Fix list_dir_begin return type

Fixes bug introduced in da4170540cbe3dcec4855ddbb59edf1abd7cf9b4.

drivers/windows/dir_access_windows.h

@@ -62,7 +62,7 @@ class DirAccessWindows : public DirAccess {
 
 public:
 
-	virtual bool list_dir_begin(); ///< This starts dir listing
+	virtual Error list_dir_begin(); ///< This starts dir listing
 	virtual String get_next();
 	virtual bool current_is_dir() const;
 	virtual bool current_is_hidden() const;