backports: add missing include for is_valid_ether_addr()

backports: add missing include for is_valid_ether_addr()

is_valid_ether_addr() is used, but the header here it is declared was
not included.

This fixes a problem introduced in commit:
commit 521acfd92f168fc9b8b6a1cefe9a165243f3d872
Author: Patrick Ziegler <patrick.ziegler@fh-kl.de>
Date:   Thu Aug 15 14:40:09 2013 +0200

    backports: add eth_prepare_mac_addr_change() and eth_commit_mac_addr_change()

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Patrick Ziegler <patrick.ziegler@fh-kl.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>