987071: specify arch of librsvg dep

On ppc64, we are installed as a .ppc package, and
gtk2 is installed as ppc/ppc64. The plain require
on librsvg2 can be satisified by the librsvg2.ppc64
package, which only provides svg loaders for the
64bit gtk2, which causes us to fail to load our
svg icons. Specify the arch of the librsvg2
dependcy needed.
Merge pull request #717 from candlepin/alikins/987071_librsvg_arch_dep

987071: specify arch of librsvg dep