netfilter: add missing xt_connlabel.h header in installation

In (c539f017

 netfilter: add connlabel conntrack extension), it
was missing the change to the Kbuild file to install the header
in the system.

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