doc-rst: fixed cleandoc target when used with O=dir

The cleandocs target won't work if I use a different output folder::

  $ make O=/tmp/kernel SPHINXDIRS="process" cleandocs
  make[1]: Entering directory '/tmp/kernel'
  make[3]: *** No rule to make target 'clean'.  Stop.
  ... Documentation/Makefile.sphinx:100: recipe for target 'cleandocs' failed
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -99,7 +99,7 @@ installmandocs:
 
 cleandocs:
 	$(Q)rm -rf $(BUILDDIR)
-	$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) -C Documentation/media clean
+	$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/media clean
 
 endif # HAVE_SPHINX