build: add a feature test for linux/net_namespace.h

This header was only introduced in 2015, so we need to build without it.

travis-ci: success for series starting with [v10,01/11] net: pass the struct nlattrs to dump() functions
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>