[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}
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