add software-properties-common

This allows using `add-apt-repository`.

Author: sigwo

doc/build-unix.md

@@ -82,6 +82,7 @@ install necessary parts of boost:
 BerkeleyDB is required for the wallet. db4.8 packages are available [here](https://launchpad.net/~bitcoin/+archive/bitcoin).
 You can add the repository and install using the following commands:
 
+    sudo apt-get install software-properties-common
     sudo add-apt-repository ppa:bitcoin/bitcoin
     sudo apt-get update
     sudo apt-get install libdb4.8-dev libdb4.8++-dev