darwin: Use usbi_get_monotonic_time() wrapper

On macOS < 10.12 we cannot use clock_gettime().

Closes #1070
diff --git a/libusb/os/darwin_usb.c b/libusb/os/darwin_usb.c
index 903422c..4fac3fd 100644
--- a/libusb/os/darwin_usb.c
+++ b/libusb/os/darwin_usb.c
@@ -1801,14 +1801,14 @@
   usbi_dbg (ctx, "darwin/reenumerate_device: waiting for re-enumeration to complete...");
 
   struct timespec start;
-  clock_gettime(CLOCK_MONOTONIC, &start);
+  usbi_get_monotonic_time(&start);
 
   while (dpriv->in_reenumerate) {
     struct timespec delay = {.tv_sec = 0, .tv_nsec = 1000};
     nanosleep (&delay, NULL);
 
     struct timespec now;
-    clock_gettime(CLOCK_MONOTONIC, &now);
+    usbi_get_monotonic_time(&now);
     UInt32 elapsed = (now.tv_sec - start.tv_sec) * 1000000 + (now.tv_nsec - start.tv_nsec) / 1000;
 
     if (elapsed >= DARWIN_REENUMERATE_TIMEOUT_US) {
diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index 6e2b30b..014b2ef 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11698
+#define LIBUSB_NANO 11699