Const modifiers

This functions do not change class variables
Would be good to mark them as const, so
class variables are not changed by coincidence

Change-Id: Iea34f6d26dbd1bde813035160e07ff2a681989e6
diff --git a/otafault/ota_io.h b/otafault/ota_io.h
index 395b423..9428f1b 100644
--- a/otafault/ota_io.h
+++ b/otafault/ota_io.h
@@ -59,7 +59,7 @@
 int ota_close(unique_fd& fd);
 
 struct OtaFcloser {
-  void operator()(FILE*);
+  void operator()(FILE*) const;
 };
 
 using unique_file = std::unique_ptr<FILE, OtaFcloser>;