Fix libbitcoin f7ac085 regression

[f7ac085](https://github.com/libbitcoin/libbitcoin/commit/f7ac085d41f1d22ceecce7bb5a680d7d3b242c93) in libbitcoin changed the stealth_prefix format. It has been updated in libwallet and sx, but not obelisk.

Not tested (due to system upgrade, will do soon).