squid: Add a PACKAGECONFIG for libnetfilter-conntrack

This solves the following warning:

WARNING: QA Issue: squid rdepends on libnetfilter-conntrack, but it isn't a
build dependency? [build-deps]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>