libnetlink: define __aligned conditionally

libnetlink: define __aligned conditionally

On some systems (e.g. current Debian/stable) the inclusion
of utils.h pulled in some other things that may end up
defining __aligned, in a possibly different way than what
we had here.

Use our own definition only if there isn't one already.

Fixes: d5acae244f9d ("libnetlink: add nl_print_policy() helper")
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>