netfilter: xt_SYNPROXY: include missing <linux/types.h>

./usr/include/linux/netfilter/xt_SYNPROXY.h:11: found __[us]{8,16,32,64} type without #include <linux/types.h>

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>