Update Mcltt.ml (#211)

avoid exit code clashing

driver/Mcltt.ml

@@ -5,7 +5,7 @@ let () =
   then begin
     Printf.fprintf stderr
       "Missing <input-file> argument.\nUsage: %s <input-file>\n" Sys.argv.(0);
-    exit 5
+    exit 7
   end;
   let filename = Sys.argv.(1) in
   let code = main_of_filename filename in