Being a pedantic jerk.

Also, [this](https://github.com/willemt/bitfield/blob/f1fd2436534e4341d7e041ce178f747d42453243/bitfield.c#L75) and [this](https://github.com/willemt/bitfield/blob/f1fd2436534e4341d7e041ce178f747d42453243/bitfield.c#L87) cause the error `error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]` to be produced when `-pedantic-errors` and `-Werror` are used ... this PR solves that.