test/fuzz: stub usbd_defer_func in net_ncm harness

The self-contained net_ncm fuzz harness #includes ncm_device.c and stubs
the usbd symbols it references rather than linking the device stack.
tud_network_link_state() now calls usbd_defer_func(), so add a matching
no-op stub to keep the harness linking.

test/fuzz/device/net_ncm/fuzz.c

diff --git a/test/fuzz/device/net_ncm/fuzz.c b/test/fuzz/device/net_ncm/fuzz.c
index a93c144..3052636 100644
--- a/test/fuzz/device/net_ncm/fuzz.c
+++ b/test/fuzz/device/net_ncm/fuzz.c
@@ -47,6 +47,9 @@
   (void) rhport; (void) p_desc; (void) ep_count; (void) xfer_type; (void) ep_out; (void) ep_in;
   return true;
 }
+void usbd_defer_func(osal_task_func_t func, void *param, bool in_isr) {
+  (void) func; (void) param; (void) in_isr;
+}
 bool tud_control_xfer(uint8_t rhport, tusb_control_request_t const *request, void *buffer, uint16_t len) {
   (void) rhport; (void) request; (void) buffer; (void) len;
   return true;