pppd/sys-linux: Fix compile with older Linux kernel headers (#452)

pppd/sys-linux: Fix compile with older Linux kernel headers (#452)

When compiling pppd against kernel headers which don't provide
the definitions for the NETLINK mechanisms, leave out the code
which uses NETLINK, so as to avoid getting compile errors.

Upstream commit in Linux refers.

commit 10c9ead9f3c6bb24bddc9a96681f7d58e6623966
Author: Roopa Prabhu <roopa@cumulusnetworks.com>
Date:   Wed Apr 20 08:43:43 2016 -0700
rtnetlink: add new RTM_GETSTATS message to dump link stats

This commit adds the #defines and structs used, so simply not compiling
this code if the required #defines isn't there should solve the problem.

Closes: #450
Signed-off-by: Jaco Kroon <jaco@uls.co.za>