Fix double prefixing issue in SMWSQLStore3Readers::getProperties

This method really should be its own class. Also not sure why this
just happened for sqlite. Perhaps the affected branch of the if is
just generally broken and another bug that is sqlite specific caused
it to be executed for sqlite?
Merge pull request #73 from SemanticMediaWiki/bugfix

Fix double prefixing issue in SMWSQLStore3Readers::getProperties