packet pool: init pthread key before using it

In the packet pool code, it's critical to initialize the pthread key
before using it. Applies only to the code used if __thread isn't
supported.

src/tmqh-packetpool.c

@@ -265,6 +265,10 @@ void PacketPoolInit(void)
 {
     extern intmax_t max_pending_packets;
 
+#ifndef TLS
+    TmqhPacketPoolInit();
+#endif
+
     PktPool *my_pool = GetThreadPacketPool();
 
     SCMutexInit(&my_pool->return_stack.mutex, NULL);