Moved threadrunner_factory to internal namespace

Author: krzikalla

include/benchmark/benchmark.h

@@ -1099,12 +1099,12 @@ struct ThreadRunnerBase {
   virtual void RunThreads(const std::function<void(int)>& fn) = 0;
 };
 
+namespace internal {
+
 // Define alias of ThreadRunner factory function type
 using threadrunner_factory =
     std::function<std::unique_ptr<ThreadRunnerBase>(int)>;
 
-namespace internal {
-
 typedef void(Function)(State&);
 
 // ------------------------------------------------------