fix build break; accomodate secp256k1 breaking API change

The present commit un-breaks the `install-sx.sh` script on the [`spesmilo/sx` repo](https://github.com/spesmilo/sx). This build break was [fixed a month ago in the version2 branch](https://github.com/libbitcoin/libbitcoin/commit/aa39fd68b9042a341ae8a50bcd5941327a55acac) by @evoskuil but it was never fixed in the master branch.  

For those of us who cannot wait for the [new libbitcoin-server](https://github.com/libbitcoin/libbitcoin-server) to be released the [only option is obelisk/master compiled against libbitcoin/master](https://github.com/libbitcoin/obelisk/issues/11#issuecomment-64822910).  The present commit should make life easier for others foolish enough to try it.