util: ignore -Wcast-align in virNetlinkDumpCommand

util: ignore -Wcast-align in virNetlinkDumpCommand

Similar to commit b202c39 ignore the warning that breaks the build
with clang:
util/virnetlink.c:365:52: error: cast from 'char *' to 'struct nlmsghdr *'
 increases required alignment from 1 to 4 [-Werror,-Wcast-align]
        for (msg = resp; NLMSG_OK(msg, len); msg = NLMSG_NEXT(msg, len)) {
                                                   ^~~~~~~~~~~~~~~~~~~~
/usr/include/linux/netlink.h:87:7: note: expanded from macro 'NLMSG_NEXT'
         (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~