iptables: correctly reference generated file

Since (14bca55 iptables: use autoconf to process .in man pages),
the file "iptables-extensions.8.tmpl" is generated from
"iptables-extensions.8.tmpl.in" and is consequently no
longer found in ${srcdir} but in the build directory.
(Becomes visible with builddir != srcdir)

Signed-off-by: Lutz Jaenicke <ljaenicke@innominate.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
1 file changed