Fix size_t not always defined.

<vector> does not provide its definition according to the standard.
However we don't actually need size_t because the number of tasks we
deal with is largely inferior to even 32-bit integers.

engine/buffered_task_scheduler.h

@@ -23,11 +23,11 @@ class BufferedTaskScheduler {
  _io_tasks.push_back(task);
  }
 
- inline size_t get_main_count() const {
+ inline unsigned int get_main_count() const {
  return _main_tasks.size();
  }
 
- inline size_t get_io_count() const {
+ inline unsigned int get_io_count() const {
  return _io_tasks.size();
  }