vfs: Make type hierarchy objects classes instead of structs

struct should be used when the data type is very simple or otherwise has
no invariants associated with it. Given these are used to form a
hierarchy, class should be used instead.

src/core/hle/service/service.h

@@ -23,7 +23,7 @@ class HLERequestContext;
 } // namespace Kernel
 
 namespace FileSys {
-struct VfsFilesystem;
+class VfsFilesystem;
 }
 
 namespace Service {