build: resolve iptables-apply not getting installed

ip6tables-apply gets installed but iptables-apply does not.
That is wrong.

» make install DESTDIR=$PWD/r
» find r -name "*app*"
r/usr/local/sbin/ip6tables-apply
r/usr/local/share/man/man8/iptables-apply.8
r/usr/local/share/man/man8/ip6tables-apply.8

Fixes: v1.8.5~87
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>