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.cpp b/otafault/ota_io.cpp
index f5b0113..3a89bb5 100644
--- a/otafault/ota_io.cpp
+++ b/otafault/ota_io.cpp
@@ -89,7 +89,7 @@
     return fclose(fh);
 }
 
-void OtaFcloser::operator()(FILE* f) {
+void OtaFcloser::operator()(FILE* f) const {
     __ota_fclose(f);
 };