net/af_iucv: clean up some forward declarations

The forward declarations for the iucv_handler callbacks are causing
various compile warnings with gcc-11. Reshuffle the code to get rid
of these prototypes.

Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>