tap: fix non-linux build

tap_fd_set_vnet_le/tap_fd_set_vnet_be was missing,
fix it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>