dumm: Fix -Wformat warning in ruby extension

dumm: Fix -Wformat warning in ruby extension

In recent ruby versions, extensions get built with -Wformat. As we use custom
printf specifiers, that does not work for us. As there does not seem to be a
reliable way to override -Wformat, we use a variable for the format string,
which prevents gcc from doing the -Wformat check in that particular situation.