configure.ac: Fix build regression on RHEL/CentOS/SL

configure.ac: Fix build regression on RHEL/CentOS/SL

This was introduced with commit 0f82228387ae ("Use more robust awk
patterns to check for backward compatibility") on RHEL 7.3+ because
it's kernel contains backported upstream commit 633c9a840d0b
("netfilter: nfnetlink: avoid recurrent netns lookups in call_batch")
that introduces @net of @struct net type parameter matched with $GREP
after $AWK returns whole @struct nfnl_callback.

This causes incorrect IPSET_CBFN() prototype choose for ->call()
of @struct nfnl_callback producing following warnings during the build:

  .../ipset/ip_set_core.c:2007:3: warning: initialization from
  incompatible pointer type [enabled by default]
     .call  = ip_set_destroy,
     ^
  ../ipset/ip_set_core.c:2007:3: warning: (near initialization
  for ‘ip_set_netlink_subsys_cb[3].call’) [enabled by default]

Fix by matching pattern to the end of first function pointer in
@struct nfnl_callback instead of end of struct.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>