bpls: fix some warnings for size_t printf arguments

Created by: germasch

%PRIu64 is for uint64_t, %zu is for size_t, which on the Mac (and 32-bit archs) aren't aliases.

[This tiny patch isn't likely to break anything since %zu is already used elsewhere in bpls.cpp, so I'd consider it safe for the release, though I think these kind of last minute changes are generally better avoided. Anyway, it's certainly not a critical fix, so it could just go into master if that's preferred.]

Mentioning @pnorbert .