USB: Do not compile USB on devices that does not have it

STM32/cores/arduino/main.cpp

@@ -47,7 +47,9 @@ int main(void)
 
 	initVariant();
 
+    #if defined(USB_BASE) || defined(USB_OTG_DEVICE_BASE)
 	USBDeviceFS.beginMenuSelection();
+    #endif
 	
 	setup();
     

STM32/cores/arduino/stm32/USBDevice.cpp

@@ -22,6 +22,8 @@
 
 #include "Arduino.h"
 
+#if defined(USB_BASE) || defined(USB_OTG_DEVICE_BASE)
+
 #include "USBDevice.h"
 #include "usbd_core.h"
 #include "usbd_desc.h"
@@ -123,3 +125,5 @@ extern "C" void USBSerial_Rx_Handler(uint8_t *data, uint16_t len){
 USBD_HandleTypeDef hUsbDeviceFS;
 
 USBDeviceClass USBDeviceFS;
+
+#endif