Append files to CLOBBER list

The rake clean and clobber tasks expect clean/clobber file list to be set via CLEAN.include and CLOBBER.include.

From rake clean's docs:

[:clean] Clean up the project by deleting scratch files and backup
files. Add files to the CLEAN file list to have the :clean
target handle them.

[:clobber] Clobber all generated and non-source files in a project.
The task depends on :clean, so all the clean files will
be deleted as well as files in the CLOBBER file list.
The intent of this task is to return a project to its
pristine, just unpacked state.

As it stands now, if one wants to hide the clobber_rdoc task: RDoc::Task.new(clobber_rdoc: ''), then the rake clobber task fails when attempting to run the unnamed clobber_rdoc task.

$ rake clobber
rake aborted!
Circular dependency detected: TOP => clobber =>  => 

If the rdoc files were added to CLOBBER directly, that would allow the clobber_rdoc task to be eliminated entirely.

Start by locating the RDoc::Task.new configuration that creates clobber_rdoc and review how the rake clobber task is assembled. Verify the change with rake clobber; done means the unnamed task no longer causes a circular dependency and the generated RDoc files are handled through CLOBBER.