tools: Fix ambiguous references to HashTable
tools: Fix ambiguous references to HashTable

Could have been GLib.HashTable or Xml.HashTable. Caught by Jenkins:
https://jenkins.qa.ubuntu.com/view/Raring/view/JHBuild%20Gnome/job/jhbuild-amd64-folks/114/

On further investigation, this was caused by changes to the libxml2
bindings in Vala. Compilation of folks against Vala master now
succeeds, so there shouldn’t be any more HashTable failures.

GNOME/folks,renatofilho/folks2,renatofilho/folks,pwithnall/folks,renatofilho/folks,pwithnall/folks,renatofilho/folks2,renatofilho/folks2,renatofilho/folks2,GNOME/folks,renatofilho/folks,GNOME/folks,renatofilho/folks,pwithnall/folks