include sys/types.h in netfilter headers exported to userspace.

When using 2.6.25 kernel headers iptables fails to compile like below
/scratch/oe/build/eglibc/tmp-omap5912osk/staging/arm-angstrom-linux-gnueabi/usr/include/linux/netfilter.h:40: error: expected specifier-qualifier-list before '__u32'
This patch includes sys/types.h in userspace netfilter headers so __u32 definition is resolved.