Fix the build with GCC after r320975

Reported by:	pfg
MFC after:	20 days
X-MFC-With:	320975
Sponsored by:	Spectra Logic Corp

tests/sys/aio/aio_test.c

@@ -222,7 +222,7 @@ setup_signal(void)
  * separate process
  */
 static struct sigevent*
-setup_thread()
+setup_thread(void)
 {
 	static struct sigevent sev;