fou6: fix proto error handler argument type

Last argument of gue6_err_proto_handler() has a wrong type annotation,
fix it and make sparse happy again.

Fixes: b8a51b38

 ("fou, fou6: ICMP error handlers for FoU and GUE")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>