fix compile error for Windows

Signed-off-by: Lyndon-Li <lyonghui@vmware.com>

pkg/uploader/kopia/block_restore_windows.go

@@ -44,3 +44,7 @@ func (o *BlockOutput) BeginDirectory(ctx context.Context, relativePath string, e
 func (o *BlockOutput) Flush() error {
 	return flushVolume(o.targetFileName)
 }
+
+func (o *BlockOutput) Terminate() error {
+	return nil
+}