add required include for usleep on Linux

On Linux, I get the following error when trying to build h3dutil:

```
lua-hapi/submodules/h3dutil/H3DUtil/src/Threads.cpp: In static member function ‘static void* H3DUtil::PeriodicThread::thread_func(void*)’:
/lua-hapi/submodules/h3dutil/H3DUtil/src/Threads.cpp:209:30: error: ‘usleep’ was not declared in this scope
```
Adding this include fixed the problem.