netfilter: ip_tables: fix build error

Fix build error introduced by commit bb70dfa5 (netfilter: xtables:
consolidate comefrom debug cast access):

net/ipv4/netfilter/ip_tables.c: In function 'ipt_do_table':
net/ipv4/netfilter/ip_tables.c:421: error: 'comefrom' undeclared (first use in this function)
net/ipv4/netfilter/ip_tables.c:421: error: (Each undeclared identifier is reported only once
net/ipv4/netfilter/ip_tables.c:421: error: for each function it appears in.)

Signed-off-by: Patrick McHardy <kaber@trash.net>