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/romfs.cpp

@@ -134,7 +134,7 @@ VirtualFile CreateRomFS(VirtualDir dir) {
         return nullptr;
 
     RomFSBuildContext ctx{dir};
-    return ConcatenateFiles(0, ctx.Build(), dir->GetName());
+    return ConcatenatedVfsFile::MakeConcatenatedFile(0, ctx.Build(), dir->GetName());
 }
 
 } // namespace FileSys