Adjust iface_khandler_register() definition to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

sys/netpfil/ipfw/ip_fw_iface.c:128:24: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
iface_khandler_register()
                       ^
                        void

This is because iface_khandler_register() is declared with a (void)
argument list, but defined with an empty argument list. Make the
definition match the declaration.

MFC after: 3 days