Enable parallel::begin<void>

[codemercenary]

Let the following code work:

for (int i : {0,4,2,5,1,3}) {
  int sleepTime = dist(mt);
  p += [i, sleepTime]() {};
}

std::vector<int> result;
for (auto it = p.begin<void(); it != p.end<void>(); ++it);

A specialization for parallel::begin<void> is needed to get this code functional.