work around xmlto bug in htmldocs

Trying to generate xhtml causes all functions to show up with a prefix
of "fsfunc" in the output, so just back off to html until someone
fixes the toolchain.

Note that this is not a problem with kernel-doc, it's an issue with
however "xmlto" renders xhtml output.
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/DocBook/Makefile

@@ -145,7 +145,7 @@ build_main_index = rm -rf $(main_idx); \
 		   cat $(HTML) >> $(main_idx)
 
 quiet_cmd_db2html = HTML    $@
-      cmd_db2html = xmlto xhtml $(XMLTOFLAGS) -o $(patsubst %.html,%,$@) $< && \
+      cmd_db2html = xmlto html $(XMLTOFLAGS) -o $(patsubst %.html,%,$@) $< && \
 		echo '<a HREF="$(patsubst %.html,%,$(notdir $@))/index.html"> \
         $(patsubst %.html,%,$(notdir $@))</a><p>' > $@