docs: also clean index.html

Missed index.html in "make cleandocs", so add it.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/DocBook/Makefile

@@ -143,7 +143,8 @@ quiet_cmd_db2pdf = PDF     $@
 	$(call cmd,db2pdf)
 
 
-main_idx = Documentation/DocBook/index.html
+index = index.html
+main_idx = Documentation/DocBook/$(index)
 build_main_index = rm -rf $(main_idx) && \
 		   echo '<h1>Linux Kernel HTML Documentation</h1>' >> $(main_idx) && \
 		   echo '<h2>Kernel Version: $(KERNELVERSION)</h2>' >> $(main_idx) && \
@@ -232,7 +233,7 @@ clean-files := $(DOCBOOKS) \
 	$(patsubst %.xml, %.pdf,  $(DOCBOOKS)) \
 	$(patsubst %.xml, %.html, $(DOCBOOKS)) \
 	$(patsubst %.xml, %.9,    $(DOCBOOKS)) \
-	$(C-procfs-example)
+	$(C-procfs-example) $(index)
 
 clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS)) man