message: Fix description message in standalone runner
diff: // Define LLVMFuzzerMutate to avoid link failures for targets that use it
  // with libFuzzer's LLVMFuzzerCustomMutator.
  extern "C" size_t LLVMFuzzerMutate(uint8_t *Data, size_t Size, size_t MaxSize) {
  - assert(false && "LLVMFuzzerMutate should not be called from afl_driver");
  + assert(false && "LLVMFuzzerMutate should not be called from standalone_fuzz_target_runner");
  return 0;
  }
  