Use the framework provided GlobalInterruptEnable() function instead of the AVR8/XMEGA specific sei() macro, for future demo/project portability.

Demos/Device/ClassDriver/KeyboardMouse/KeyboardMouse.c

@@ -93,7 +93,7 @@ int main(void)
 	SetupHardware();
 
 	LEDs_SetAllLEDs(LEDMASK_USB_NOTREADY);
-	sei();
+	GlobalInterruptEnable();
 
 	for (;;)
 	{