Added missing route to `SubsiteXHRController` for SilverStripe 3.2 compatibility.
Added missing route to `SubsiteXHRController` for SilverStripe 3.2 compatibility.

silverstripe/silverstripe-subsites,danaenz/silverstripe-subsites,mandrew/silverstripe-subsites,silverstripe/silverstripe-subsites,mikenz/silverstripe-simplesubsites,mandrew/silverstripe-subsites,mikenz/silverstripe-simplesubsites,danaenz/silverstripe-subsites