Fix a type error

[brianmwaters]

Project was not building for me w/ GCC 9.2.0 and Boost 1.71.0. Casting this to a long fixed the problem. I have not tested it at runtime; in particular, I do not know if the number is typically less than 1, or much more than 1 in real usage. (If it is much more than 1, then this patch will be fine, otherwise it may be problematic.)

[brianmwaters]

I see now that this is basically a duplicate of https://github.com/balint256/gr-baz/pull/56