Fixes the following build failure:

  CC      arch/sparc/kernel/sys_sparc32.o
In file included from include/linux/netfilter_ipv4/ip_tables.h:28,
                 from arch/sparc/kernel/sys_sparc32.c:46:
include/linux/netfilter/x_tables.h:525: error: expected declaration specifiers or ‘...’ before ‘nf_hookfn’
Signed-off-by: David S. Miller <davem@davemloft.net>