[Future] Fix invalid conversion from unsigned to ptr

Fix

emper/io/Future.cpp:184:35: error: invalid conversion from ‘uint64_t’ {aka ‘long unsigned int’} to ‘void*’ [-fpermissive] 184 | io_uring_prep_cancel(sqe, user_data, 0); | ^~~~~~~~~ | | | uint64_t {aka long unsigned int}