doc: update doc/sphinx/kerneldoc.py

Update doc/sphinx/kerneldoc.py from Linux next-20200219 to avoid warnings
like:

doc/sphinx/kerneldoc.py:125: RemovedInSphinx20Warning:
AutodocReporter is now deprecated. Use
sphinx.util.docutils.switch_source_input() instead.
  self.state.memo.reporter =
  	AutodocReporter(result, self.state.memo.reporter)

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>