AutoYaST: rubygem-cstruct needs to be installed

The crowbar ohai plugin depends on rubygem-cstruct nowadays. Installing it as
part of the ohai recipe in the deployer barclamp is not easily possible as that
recipe is also executed inside sle(s|dge)hammer and we can't install packages
at that stage.
Merge pull request #130 from rhafer/release/pebbles/master

AutoYaST: rubygem-cstruct needs to be installed