BUILD/MINOR: threads: always export thread_sync_io_handler()

Otherwise it doesn't build again without threads.
diff --git a/src/hathreads.c b/src/hathreads.c
index 0d0b376..839b086 100644
--- a/src/hathreads.c
+++ b/src/hathreads.c
@@ -21,6 +21,11 @@
 THREAD_LOCAL unsigned int tid      = 0;
 THREAD_LOCAL unsigned long tid_bit = (1UL << 0);
 
+/* Dummy I/O handler used by the sync pipe.*/
+void thread_sync_io_handler(int fd)
+{
+}
+
 #ifdef USE_THREAD
 
 static HA_SPINLOCK_T sync_lock;
@@ -32,11 +37,6 @@
 struct lock_stat lock_stats[LOCK_LABELS];
 #endif
 
-/* Dummy I/O handler used by the sync pipe.*/
-void thread_sync_io_handler(int fd)
-{
-}
-
 /* Initializes the sync point. It creates a pipe used by threads to wakup all
  * others when a sync is requested. It also initialize the mask of all create
  * threads. It returns 0 on success and -1 if an error occurred.