vfs_concat/vfs_layered: Remove friend declarations from ConcatenatedVfsFile

Given these are only added to the class to allow those functions to
access the private constructor, it's a better approach to just make them
static functions in the interface, to make the dependency explicit.

src/core/file_sys/registered_cache.cpp

@@ -125,7 +125,7 @@ VirtualFile RegisteredCache::OpenFileOrDirectoryConcat(const VirtualDir& dir,
             if (concat.empty())
                 return nullptr;
 
-            file = FileSys::ConcatenateFiles(concat, concat.front()->GetName());
+            file = ConcatenatedVfsFile::MakeConcatenatedFile(concat, concat.front()->GetName());
         }
 
         return file;