USB_Init() no longer calls sei() to enable global interrupts - this must now be done in the user application once all init code has run.

Demos/Host/ClassDriver/MouseHost/MouseHost.c

@@ -65,6 +65,7 @@ int main(void)
 	puts_P(PSTR(ESC_FG_CYAN "Mouse Host Demo running.\r\n" ESC_FG_WHITE));
 
 	LEDs_SetAllLEDs(LEDMASK_USB_NOTREADY);
+	sei();
 
 	for (;;)
 	{

Demos/Host/ClassDriver/MouseHost/MouseHost.h

@@ -41,6 +41,7 @@
 		#include <avr/wdt.h>
 		#include <avr/pgmspace.h>
 		#include <avr/power.h>
+		#include <avr/interrupt.h>
 		#include <stdio.h>
 
 		#include <LUFA/Version.h>