Fix NAT stress test time helper on Windows

tests/test_nat_stress.c

@@ -33,7 +33,7 @@ static struct {
 #define ASSERT_EQ(a,b,m) ASSERT((a)==(b),m)
 
 static double now_ms(void) {
-    struct timeval tv; gettimeofday(&tv, NULL);
+    struct timeval tv; utun_gettimeofday(&tv, NULL);
     return tv.tv_sec * 1000.0 + tv.tv_usec / 1000.0;
 }