Update build-unix.md
Update build-unix.md

* Added `software-properties-common` so add-apt-repository works
* Changed link to bitcoin vice syscoin, which is non-existent (for ppa:bitcoin/bitcoin and the link to launchpad.net)

Former-commit-id: 9820653b40129ef256e3b3bd15cf9481c7161110
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin