Document openscap lib required for openscap gem to work

Unusually, since the gem uses FFI at runtime, you don't need this lib
for `bundle install` to succeed, but `require 'openscap'` will fail.

developer_setup.md

@@ -26,6 +26,7 @@
  rpm -q --whatprovides npm || sudo dnf -y install npm # For CentOS 7, Fedora 23 and older
  sudo dnf -y install openssl-devel # For rubygems
  sudo dnf -y install cmake # For rugged Gem
+ sudo dnf -y install openscap # For openscap Gem
  ```
 
 * Install the _Bower_ package manager