pstats: Fix compilation without HAVE_THREADS or with SIMPLE_THREADS

panda/src/pstatclient/pStatClientImpl.I

@@ -91,6 +91,7 @@ client_resume_after_pause() {
   _delta -= delta;
 }
 
+#if defined(HAVE_THREADS) && !defined(SIMPLE_THREADS)
 /**
  *
  */
@@ -99,3 +100,4 @@ QueuedFrame(int thread_index, int frame_number) :
   _thread_index(thread_index),
   _frame_number(frame_number) {
 }
+#endif