add const to the pointer arg to h_make_bytes
cf. https://github.com/UpstandingHackers/hammer/pull/196
got a compiler complaint in my PDF parser from this: can't use h_make_bytes() on arrays you only have a const handle on.
i think this was probably a simple oversight in the type (the field in HBytes is const anyway), but my mind is elsewhere right now, so I'm leaving it as an open pull request for the moment.