use the internal macro name FUTEX_PRIVATE in __wait

the name was recently added for the setxid/synccall rework,
so use the name now that we have it.

src/thread/__wait.c

@@ -3,7 +3,7 @@
 void __wait(volatile int *addr, volatile int *waiters, int val, int priv)
 {
 	int spins=100;
-	if (priv) priv = 128;
+	if (priv) priv = FUTEX_PRIVATE;
 	while (spins-- && (!waiters || !*waiters)) {
 		if (*addr==val) a_spin();
 		else return;