network: selectively disable -Wcast-align in virNetDevParseDadStatus

Commit 0f7436ca "network: wait for DAD to finish for bridge IPv6 addresses"
results in:

 CC     util/libvirt_util_la-virnetdevmacvlan.lo
util/virnetdev.c: In function 'virNetDevParseDadStatus':
util/virnetdev.c:1319:188: error: cast increases required alignment of target type [-Werror=cast-align]
util/virnetdev.c:1332:41: error: cast increases required alignment of target type [-Werror=cast-align]
util/virnetdev.c:1334:92: error: cast increases required alignment of target type [-Werror=cast-align]
cc1: all warnings being treated as errors

on at least ARM platforms.

The three macros involved (NLMSG_NEXT, IFA_RTA and RTA_NEXT) all appear to
correctly take care of alignment, therefore suppress Wcast-align around their
uses.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Maxim Perevedentsev <mperevedentsev@virtuozzo.com>
Cc: Laine Stump <laine@laine.org>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>