[Hist] Disable autoloading of unsupported ROOT7 THist variadic templates

Avoids warnings like the ones below by rootcling:

Warning: Template parameter pack found: autoload of variadic templates is not supported yet.
Warning: Problems with arguments for forward declaration of class THist.

Reference on CDash: http://cdash.cern.ch/viewBuildError.php?type=1&buildid=375697