fix pthread_kill unlocking

it had not been updated for the futex-based locks

src/thread/pthread_kill.c

@@ -5,6 +5,6 @@ int pthread_kill(pthread_t t, int sig)
 	int r;
 	__lock(&t->killlock);
 	r = t->dead ? ESRCH : -__syscall(SYS_tgkill, t->pid, t->tid, sig);
-	a_store(&t->killlock, 0);
+	__unlock(&t->killlock);
 	return r;
 }