travis: Disable check for array of flexible structures in sparse.

travis: Disable check for array of flexible structures in sparse.

Sparse introduced new checks for flexible arrays and there is a
false-positive in netdev-linux implementation right now that can not
be easily fixed.  Patch sent to sparse to fix it, but we need to
disable the check for now to unblock our CI.

  lib/netdev-linux.c:1238:19: error: array of flexible structures

The issue is with the following code:

  union {
      struct cmsghdr cmsg;
      char buffer[CMSG_SPACE(sizeof(struct tpacket_auxdata))];
  } cmsg_buffers[NETDEV_MAX_BURST];

'struct cmsghdr' contains a flexible array.  But this union is a way
to ensure correct alignment of 'buffer', suggested by CMSG manpage.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>