Add missing USBCON wrapper

Author: sandeepmistry

Diff for: cores/arduino/USB/USBCore.cpp

@@ -16,6 +16,8 @@
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
+#if defined(USBCON)
+
 #include <Arduino.h>
 
 #include "SAMD21_USBDevice.h"
@@ -968,3 +970,4 @@ void USBDeviceClass::ISRHandler()
 // USBDevice class instance
 USBDeviceClass USBDevice;
 
+#endif