doc-rst: make cleandocs misses a fair number of files

Removes intermediate 'Documentation/DocBook/.*.xml.cmd' files

Changes since v1:
- Reduce the patch to DocBook cleandocs

References: http://lkml.kernel.org/r/CA+r1Zhjr5SCVAroREBv84t9bxDVu5jVJ_Fu=BbVDGNNABdQOuQ@mail.gmail.comReported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -264,6 +264,7 @@ clean-files := $(DOCBOOKS) \
 	$(patsubst %.xml, %.aux.xml, $(DOCBOOKS)) \
 	$(patsubst %.xml, %.xml.db,  $(DOCBOOKS)) \
 	$(patsubst %.xml, %.xml,     $(DOCBOOKS)) \
+	$(patsubst %.xml, .%.xml.cmd, $(DOCBOOKS)) \
 	$(index)
 
 clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS)) man