nanostack-hal: eventloop: silence compiler warning for unused variable

Compiler reminded that a variable declaration was left behind when
the code using it was put behind #ifdef. Add the missing #ifdef.

Warning being fixed:
---8<---8<----
[Warning] ns_event_loop.c@44,0:  #177-D: variable "event_thread_id"
was declared but never referenced

Author: TeroJaasko

features/FEATURE_COMMON_PAL/nanostack-hal-mbed-cmsis-rtos/ns_event_loop.c

@@ -41,7 +41,10 @@ static const osThreadAttr_t event_thread_attr = {
 };
 #endif
 
+#if !MBED_CONF_NANOSTACK_HAL_EVENT_LOOP_DISPATCH_FROM_APPLICATION
 static osThreadId_t event_thread_id;
+#endif
+
 static mbed_rtos_storage_mutex_t event_mutex;
 static const osMutexAttr_t event_mutex_attr = {
   .name = "nanostack_event_mutex",