Sign conversion warning in fifo.tcc

[drowaudio]

The conversion from an int to a size_t here causes a warning on clang: https://github.com/hogliux/farbot/blob/489ab870e41de7b7f569540e67263c976a757ea0/include/farbot/detail/fifo.tcc#L177

Resolution: slots (static_cast<size_t> (capacity))