Fix win32 build of PThread

Summary:
This fails to build on win32 platform with:
```
xplat\folly\portability\pthread.cpp(142): error C2664: 'HANDLE CreateThread(LPSECURITY_ATTRIBUTES,SIZE_T,LPTHREAD_START_ROUTINE,LPVOID,DWORD,LPDWORD)': cannot convert argument 3 from 'DWORD (__cdecl *)(void *)' to 'LPTHREAD_START_ROUTINE'
```

This seems to fix it.

Reviewed By: Orvid

Differential Revision: D21623386

fbshipit-source-id: 1b18a74bd71ffc75f34a92074e07f3d69742c07d

folly/portability/PThread.cpp

@@ -100,7 +100,7 @@ struct pthread_startup_info {
   void* startupArgument;
 };
 
-DWORD internal_pthread_thread_start(void* arg) {
+DWORD __stdcall internal_pthread_thread_start(void* arg) {
   // We are now in the new thread.
   auto startupInfo = reinterpret_cast<pthread_startup_info*>(arg);
   current_thread_self = startupInfo->thread;