Bluetooth: Add BUILD_BUG_ON check for SKB control buffer size

The struct bt_skb_cb size needs to stay within the limits of skb->cb
at all times and to ensure that add a BUILD_BUG_ON to check for it at
compile time.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>