Pass FPSmanager size as size_t to malloc instead of u64.

Changes the FPSmanager constructor to use the architecture dependent
size_t type from the libc crate instead of u64 as size when allocating
the memory for the instance.

Note: I'm not very versed in this kind of low-level bindings, this might be
the wrong approach. It makes it compile on my 32bit machine, but that
might not mean much