USBCDC: add doxy only to provide doxygen for this object always

Author: 0xc0170

drivers/USBCDC_ECM.h

@@ -18,7 +18,7 @@
 #ifndef USBCDC_ECM_H
 #define USBCDC_ECM_H
 
-#if defined(MBED_CONF_RTOS_PRESENT)
+#if defined(MBED_CONF_RTOS_PRESENT) || defined(DOXYGEN_ONLY)
 #include "USBDescriptor.h"
 #include "USBDevice.h"
 #include "ByteBuffer.h"