[simix] Fix signatures of callbacks (timer and kill_process_function)

[simix] Fix signatures of callbacks (timer and kill_process_function)

* Conversion between pointer-to-pointer and pointer to data is
  undefined behaviour.

* Moreover, this hides error. The kill_process_function was called in
  the timer with only one argument whreas it expects two arguments
  (the killed process and the killer process).

* Simpligy the signature of SIMIX_function_register_process_kill() to
  only take a single argument. The second one is never used.