3ds: Fix crash on shutdown
It is required to call gfxExit() before exiting the app if gfxInit() was previously called. Otherwise, the GSP thread continues to run after the stack is invalidated. The missing code is already in lcd_shutdown(), but it was never called because HAVE_LCD_SHUTDOWN was not defined for the ctru target.
Change-Id: I8999df6372cd593c5b52478028ad7421b23d5f92