USB: composite: usb_composite_unregister() no longer __exit

Changed definition of usb_composite_unregister() function
removing __exit declaration.  This way, the function is
included even if the whole code was not compiled as module.
This is required if a compiled-in code would like to
unregister a composite gadget.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>