Kernel: Fix various forward declarations

I decided to modify MappedROM.h because all other entried in Forward.h
are also classes, and this is visually more pleasing.

Other than that, it just doesn't make any difference which way we resolve
the conflicts.

Kernel/FileSystem/VirtualFileSystem.h

@@ -43,7 +43,7 @@ namespace Kernel {
 class Custody;
 class Device;
 class FileDescription;
-class UnveiledPath;
+struct UnveiledPath;
 
 struct UidAndGid {
     uid_t uid;