netfilter: xtables: remove __pure annotation

sparse complains:
ip_tables.c:361:27: warning: incorrect type in assignment (different modifiers)
ip_tables.c:361:27:    expected struct ipt_entry *[assigned] e
ip_tables.c:361:27:    got struct ipt_entry [pure] *

doesn't change generated code.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>