ofproto-dpif-xlate: Suppress some warnings on non-Linux OSes

These warnings were introduced by
commit 7d031d7e511aeea8dd45348922fe8e3bbdd2956e
("ofproto-dpif-xlate: Work around Linux netdev_max_backlog limit.")
and found by --enable-Werror build on NetBSD.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>